Cheol-Hoon Park

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.

Design of a simple, lightweight, passive-elastic ankle exoskeleton supporting ankle joint stiffness

In this study, a passive-elastic ankle exoskeleton (PEAX) with a one-way clutch mechanism was developed and then pilot-tested with vertical jumping to determine whether the PEAX is sufficiently lightweight and comfortable to be used in further biomechanical studies. The PEAX was designed to supplement the function of the Achilles tendon and ligaments as they passively support the ankle torque with their inherent stiffness. The main frame of the PEAX consists of upper and lower parts connected to each other by tension springs (N = 3) and lubricated hinge joints. The upper part has an offset angle of 5° with respect to the vertical line when the springs are in their resting state. Each spring has a slack length of 8 cm and connects the upper part to the tailrod of the lower part in the neutral position. The tailrod freely rotates with low friction but has a limited range of motion due to the stop pin working as a one-way clutch. Because of the one-way clutch system, the tension springs store the elastic energy only due to an ankle dorsiflexion when triggered by the stop pin. This clutch mechanism also has the advantage of preventing any inconvenience during ankle plantarflexion because it does not limit the ankle joint motion during the plantarflexion phase. In pilot jumping tests, all of the subjects reported that the PEAX was comfortable for jumping due to its lightweight (approximately 1 kg) and compact (firmly integrated with shoes) design, and subjects were able to nearly reach their maximum vertical jump heights while wearing the PEAX. During the countermovement jump, elastic energy was stored during dorsiflexion by spring extension and released during plantarflexion by spring restoration, indicating that the passive spring torque (i.e., supportive torque) generated by the ankle exoskeleton partially supported the ankle joint torque throughout the process.

Effects of Preheat Supply on Embossed Pattern Depth in Roll-to-Roll Process

In this study, we examined the sensitivity of embossed pattern depth to preheat supply and cooling, and also tested how pattern type and density affect the embossed depth. The main factors such as roller temperature, roller speed, and applied force that mostly affect embossed pattern qualities of roll-to-roll hot embossing were determined based on the response surface methodology. Eight conditions were then added to know the time-dependent effects of heat transfer with custom designed preheating and cooling systems (Fig. 1). Extended preheat time for the PMMA substrate contributed to the significant change of the embossed depth, whereas the substrate-cooling showed no straightforward increasing or decreasing trend. Larger embossed depths were achieved in horizontal patterns with lower density compared to vertical patterns, and the lower pattern densities showed the greater embossed depths regardless of pattern types. We expect that this result may help to understand the effects of preand post-treatment of roll-to-roll (R2R) hot embossing by employing time duration factors of heat transfer, depending on mold pattern type and density.

Stability analysis of a magnetically-levitated large flywheel using flexible rotor modeling

Stability analysis of the flywheel energy storage system supported by active magnetic bearings is studied in this paper. The system is designed to store 5kWh at maximum speed of 18,000 rpm and the gyroscopic coupling is strong that the dynamics of system changes with respect to the running speed. To analyze the stability of the system accurately, we derived mathematical models for the system components including the rotor, active magnetic bearings, amplifiers and sensors. The rotor model is based either on rigid-body dynamics or flexible rotor model using finite-element approach. We checked the stability of the system using the derived models and found that the model based rigid-body dynamics can be misleading, and an accurate rotor model using flexible rotor dynamics must be used in order to predict the stability properly.

Analysis of a Novel MR Rotary Brake with Permanent Magnet

In this paper, a novel MR brake with permanent magnet is developed. This system consists of rotary disk, permanent magnet, spring and MR fluid. Permanent magnets are attached to the rotary disk and moves in the direction of radius. The magnets are linked to rotor axis by spring. As rotation speed increases, the magnets move outward from the center of the system by centrifugal force in the MR fluid. A proper design of stator or case makes the system have unique torque characteristics. The research is performed like following procedures. First, the electromagnetic characteristic of the system is analyzed using Maxwell. Next, torque is calculated using the results of the electromagnetic analysis. Finally, the performance of various types of the brake systems are investigated and compared with each other.