Seong-yeol Yoo

Optimal design of micro flywheel energy storage system

We have designed a micro flywheel energy storage system in which the flywheel stores electrical energy in terms of kinetic energy and converts this kinetic energy into electrical energy when necessary. The flywheel is supported by two radial permanent magnet passive bearings. Permanent magnet passive bearings use the repulsive forces between two sets of permanent magnets so that contact free rotation is possible. A set of voice coil actuator provides the axial stability, resulting in a complete magnetic levitation. The toroidally-wound BLDC motor which has high efficiency and little additional negative radial stiffness is used for motoring and generation. Micro flywheel energy storage system is optimally designed to have the maximum energy storage capacity. In this paper, we present the design equation for the components in a micro flywheel energy storage system and the optimal design process for these components..

Toroidally-Wound Self-Bearing BLDC Motor With Lorentz Force

Self-bearing motors (SBM) use a single magnetic structure for rotational motoring as well as for noncontact levitation. They are sometimes referred to as bearingless motors or combined motor-bearings. In this paper, we propose a new type of self-bearing motors based on toroidally-wound brushless DC machines. This type of SBM can be made to be passively stable in the axial direction and for out-of-plane rotations. To achieve self-bearing operation, we derive a force-current model and show that the levitation force is decoupled from the rotational torque. To overcome the singularity problem in the force-current model, we propose a phase selection algorithm in which the phase that may cause singularity is counted out when inverting the force-current model. Through finite element analyses and experiments, we validate the force-current model and the phase selection algorithm.

Validation of Flexible Rotor Model for a Large Capacity Flywheel Energy Storage System

When we design a controller for the active magnetic bearings that support a large rotor, it is important to have an accurate model of the rotor. For the case of the flywheel that is used to store energy, an accurate rotor model is especially important because the dynamics change with respect to the running speed due to gyroscopic effects. In this paper, we present a procedure of obtaining an accurate rotor model of a large flywheel energy storage system using finite-element method. The model can predict the first and the second bending mode which match well with the experimental results obtained from a prototype flywheel energy storage system.

Analysis and Modeling of a Voice-Coil Linear Vibration Motor Using the Method of Images

The vibration motors for smart phones with haptic interfaces require higher reaction rate and longer service life than what a typical rotational motor with eccentric mass provides. In this paper, a new type of a linear vibration motor is proposed. A voice-coil actuator is employed for high reaction rate. Permanent magnets replace mechanical springs, which would help increase the service life. For analyzing and modeling the proposed vibration motor, we use the method of equivalent current sheet and the method of images. These methods are validated against finite element analyses and experiments. A prototype motor is designed and fabricated. Tests with the prototype show the feasibility of the proposed linear vibration motor.

Finite Element Analysis of CFRP Frame under Launch and Recovery Conditions for Subsea Walking Robot, Crabster

This study applied finite element analysis (FEA) to the body frame of the 200-meter class multi-legged subsea walking robot known as Crabster (CR200). The body frame of the CR200 is modeled after the ribcage of a human so that it can disperse applied external loads. It is made of carbon-fiber-reinforced plastic (CFRP). Therefore, the frame is lighter and stronger than it would be if it were made of other conventional materials. In order to perform FEA for the CFRP body frame, we applied the material properties of the CFRP as obtained from a specimen test to an FE model of CFRP frame. Finally, we performed FEA with respect to the load conditions encountered when the robot is launched into and recovered from the sea. Also, we performed FEA for the frame, assuming that it was fabricated using a conventional material, in order to compare its characteristics with CFRP.

Finite Element Analysis of Carbon Fiber Reinforced Plastic Frame for Multi-legged Subsea Robot

This paper describes a finite element analysis (FEA) of the body frame of a subsea robot, Crabster200 (CR200). CR200 has six legs for mobility instead of screw type propellers, which distinguishes it from previous underwater robots such as remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs). Another distinguishing characteristic is the body frame, which is made of carbon fiber reinforced plastic (CFRP). This body frame is designed as a rib cage structure in order to disperse the applied external loads and reduce the weight. The frame should be strong enough to support many devices for exploration and operation underwater. For a reasonable FEA, we carried out specimen tests. Using the obtained material properties, we performed a modal analysis and FEA for CR200 with a ready posture. Finally, this paper presents the FEA results for the CFRP body frame and the compares the characteristics of CFRP with conventional material, aluminum.

Comparative study of power minimizing control algorithms for active magnetic bearings

This paper describes a comparative study of power minimizing control algorithms for active magnetic bearing (AMB) systems. Active magnetic bearings usually employ bias currents in order to linearize the nonlinear force-current relationship. Although these bias currents guarantee certain amount of dynamic performance, ohmic losses due to the bias currents are a major source of power consumption in a magnetic bearing system. There have been efforts to reduce or eliminate the bias currents while satisfying the requirements. However, these power minimizing algorithms have never been compared against each other under same conditions. In this paper, we compare the two of the existing power minimizing algorithms with a proportional-derivative (PD) type controller which is a de facto controller for magnetic bearings. Also, a PD controller with synchronous notch filter is compared with the power minimizing controllers in a attempt to appraise the performance of these controllers as well as the power saving.

Optimal Notch filter for active magnetic bearing controllers

This paper describes an optimal design of a notch filter for the active magnetic bearing (AMB) system which has unstable poles. A Notch filter is designed as 2nd order standard transfer function with two zeros and two poles. We performed optimization to design a notch filter using sequential quadratic programming (SQP) and evaluate the closed loop stability with a designed notch filter. The designed notch filter is optimized to have the lowest peak sensitivity with respect to the disturbance closed loop system. The designed notch filter is examined for the compensation effect for the unstable 2nd order system. We applied the design optimization to large scale flywheel energy storage system (FESS). FESS can store up to 5kWh of usable energy at the maximum speed of 18,000 rpm. The effect of the designed filter from optimization is simulated using simulation model of FESS and the simulation model is validated against experiments.

Measurement of hydrodynamic forces and moment acting on Crabster, CR200 using model tests

Crabster CR200 is a remotely operated vehicle which has six artificial and articulated legs driven by BLDC motors. It is supposed to walk on the sea floor. As a result, the robot has a very complicated appearance and hydrodynamic forces and moment acting on the robot experience a variety of changes according to configurable legs and body postural position. This paper describes the experiments to estimate lift force, drag force and pitching moment acting on CR200 using a 1/4 length ratio scaled model. A 6-axis Force/Torque sensor was adopted and installed in the middle of inner space of the model to measure the forces and moment. The experiments were conducted in a circulating water channel whose width and depth are 3.5m and 3.0m, respectively. To define hydrodynamic characteristics effected by the floor, the authors approached in two ways. The first is that CR200 was assumed to stand on the ground. The other is when the robot is placed in the middle of stream of water flow.

Multi-legged ROV Crabster and an acoustic camera for survey of underwater cultural heritages

In west coast of Korea peninsula, there are some offshore sites where ancient sunken shipwrecks have been discovered. These ships are supposed to carry an amount of artifacts what show lifestyle and culture of the day. In these areas, however, because of fast ocean currents and low visibility, underwater survey and excavation by divers are strictly restricted. Especially, optical cameras and humans visual inspection become defective. To overcome these conditions, use of unmanned underwater vehicles such like a ROV(Remotely Operated Vehicle) which is equipped with acoustic imaging devices has been raised. This paper describes preliminary performance experiments for survey of underwater cultural heritages using a ROV whose name is Crabster driven by six artificial legs with a high-resolution acoustic camera in sea. The Crabster uses the six articulated legs actuated by BLDC motors as its thrusters. Therefore, it moves on the sea floor without disturbances by propellers which are widely used as thrusters of any other general ROVs. In addition, The robot is expected to minimize shadow areas of acoustic imaging sensors by controlling its body posture precisely using the legs. In this paper, we introduce operations of Crabster and the acoustic imaging devices when we conducted from April to May in 2015. Using acoustic imaging devices, we could find dispersed potteries and iron caldrons. Mosaicked image to show surround of the robot is also included.