Chang-Hyuk Lee

In-Plane Thermal Conductivity of Polycrystalline Chemical Vapor Deposition Graphene with Controlled Grain Sizes

Manipulation of the chemical vapor deposition graphene synthesis conditions, such as operating P, T, heating/cooling time intervals, and precursor gas concentration ratios (CH4/H2), allowed for synthesis of polycrystalline single-layered graphene with controlled grain sizes. The graphene samples were then suspended on 8 μm diameter patterned holes on a silicon-nitride (Si3N4) substrate, and the in-plane thermal conductivities k(T) for 320 K < T < 510 K were measured to be 2660-1230, 1890-1020, and 680-340 W/m·K for average grain sizes of 4.1, 2.2, and 0.5 μm, respectively, using an opto-thermal Raman technique. Fitting of these data by a simple linear chain model of polycrystalline thermal transport determined k = 5500-1980 W/m·K for single-crystal graphene for the same temperature range above; thus, significant reduction of k was achieved when the grain size was decreased from infinite down to 0.5 μm. Furthermore, detailed elaborations were performed to assess the measurement reliability of k by addressing the hole-edge boundary condition, and the air-convection/radiation losses from the graphene surface.

Designing a Permanent-Magnetic Actuator for Vacuum Circuit Breakers Using the Taguchi Method and Dynamic Characteristic Analysis

Permanent-magnetic actuators (PMAs) are usually used for vacuum circuit breakers (VCBs) due to their fast linear motion and large holding force. As the holding force becomes larger, so does the current capacity of the circuit breaker. Producing a large holding force with the same PMA size is therefore important in PMA design. This paper presents a design method of a PMA for VCBs. To attain the desired holding force under the restriction of the PMA size, most of the influential PMA yoke design parameters were obtained by the Taguchi method. PMA dynamic characteristics were simulated considering the electromagnetic characteristics, the PMA driving circuit, and the mechanical components connected to the PMA output shaft. Experiments were performed to validate the proposed design method, and the experimental results were compared with the simulation results.

Initial positioning of a smart actuator using dual absolute encoders

In order to precisely control motor positions, most motor systems use an encoder as the feedback sensor. In the case of a simple system, an incremental encoder and limit switches are used for initial position setting. However, this system loses the current position data when the power goes down and the final position is changed due to an external force. For this reason, absolute encoders are used in full-fledged systems and to facilitate the best performance, a high-resolution absolute encoder should be directly attached at the output axis. However high-resolution absolute encoders are usually large and expensive, and in many cases, it is structurally difficult to directly install the absolute encoder to the output axis. An alternative to this direct sensing of the output angle is a multiturn absolute encoder, which is attached to the motor shaft. However, the multiturn absolute encoder also loses its position data when the backup battery runs out and an external force is applied. In this paper, we therefore suggest a dual absolute encoder system, in which one absolute encoder is attached to the motor shaft and the other low-resolution absolute encoder is attached to the output shaft of the harmonic drive speed reducer (the output shaft of a smart actuator). The required conditions for the errorless operation of this dual encoder system are derived and verified by simulations and experiments.

A compact stair-climbing wheelchair with two 3-DOF legs and a 1-DOF base

Purpose – The purpose of this paper is to describe a compact wheelchair, which has two 3-degrees of freedom (DOF) legs and a 1-DOF base (the total DOF of the leg system is 7) for stair-climbing, and wheels for flat surface driving. Design/methodology/approach – The proposed wheelchair climbs stairs using the two 3-DOF legs with boomerang-shaped feet. The leg mechanisms are folded into the compact wheelchair body when the wheelchair moves over flat surfaces. The authors also propose a simple estimation method of stair shape using laser distance sensors, and a dual motor driving system to increase joint power. Findings – The proposed wheelchair can climb arbitrary height and width stairs by itself, even when they are slightly curved. During climbing, the trajectory of the seat position is linear to guarantee the comfort of rider, and the wheelchair always keeps a stable condition to ensure the stability in an emergency stop. Originality/value – The wheelchair mechanism with foldable legs and driving wheels ...

Power-Assisted Wheelchair With Gravity and Friction Compensation

In this paper, a gravity compensated power-assisted wheelchair (GCPAW) is proposed, which will climb a hill, and stop, and make as a delicate movement in the middle of the hill as it can do on a flat surface. To climb the hill, the proposed GCPAW compensates for the parallel component of the force of gravity (the force that causes acceleration or deceleration) on a hill. A tilt sensor measures the angle of the inclined plane and electric clutches automatically connect and disconnect drive mechanisms with the wheels. The GCPAW also compensates for friction in the drive mechanism that would cause additional resistance. The rider propels the GCPAW in the same way that he/she would propel a manual wheelchair, but the mechanism compensates for both gravity and the friction. Additionally, overcompensation for the friction helps the riders propulsion, so that with the same propulsion force the rider travels further than in a manual wheelchair, resulting in an easier movement for the rider.

Design of a high-speed, short-stroke xy-stage with counterbalance mechanisms

In many cases, the moving speed of xy-stages influences the productivity of industrial machines; thus, it is very important to develop high speed xy-stages. This paper presents a high-speed/short-stroke xy-stage mechanism, which is used on a long-stroke stage and which performs high-speed xy-motions in a short range. To reduce the weight of the moving part, the proposed xy-stage transfers the motion of two actuators to the stage through two perpendicularly-installed linear motion guides. The dump force applied to the base frame is also greatly reduced by adding the counterbalance mechanism. A prototype design and its simulation results are presented.

AUTOMATIC C-ARM REPOSITIONING USING A TWO-BAR LINK SYSTEM FOR REDUCING RADIATION EXPOSURE

A C-arm fluoroscopy is a useful instrument for finding out the position of a needle in a minimally invasive surgery, but its use means that the surgeon and patient are exposed to radiation while the surgeon places the C-arm into the previous position and orientation as well as he/she regularly uses it to take X-ray images. In this paper, a two-bar link system and moving mechanism added on a C-arm are proposed to reduce the dose of the exposure. The two-bar link system consists of two bar links and three absolute encoders. It measures the relative position and angle between the C-arm and the surgical table in real time and stores the measurements in the system. The moving platform of C-arm is proposed to have two switchable casters for both passive freewheeling and active motor driving. Passive freewheeling allows the C-arm to be freely moved by human force and active motor driving returns the C-arm to its original position automatically. A simple experiment is performed to verify the feasibility of C-arm repositioning using a two-bar link system.