Jo Sep 15, 2024
Permanent magnet (PM) brushless DC (BLDC) motors are widely used in various applications demanding speed regulation and control. PMBLDC motors are a typical example with advantages of high efficiency, high power density and maintenance-free operation. However, in some applications with limited space, high temperature or severe vibrations, use of Hall-effect sensors for rotor position detection may encounter hazards or problems. For example, high temperature can affect the characteristics or cause failure of sensors while insufficient space would not be able to accommodate sensors. Other conditions such as severe vibration or in-water operation may involve a risk of sensor destruction. Therefore, sensorless control is considered as an appropriate technique for such applications.
Sensorless control can reduce not only the number of external wiring/connections between the motor and the driver but also the maintenance requirement caused by vibration. This will improve the system reliability and reduce cost.
There have been a lot of studies reporting sensorless control techniques for BLDC motors. Some of them are based on detection of zero crossing points (ZCPs) of back electromotive force (EMF) or terminal voltages. The ZCP techniques often require the voltage of the neutral point or virtual neutral point, which are used for comparison with terminal voltages for commutation. However, neutral or virtual neutral points often contain high-frequency noise and windings of common motors may have no lead-outs for neutral points.
To avoid this problem, some studies indirectly obtained back-EMF ZCPs with a series of signal subtractions and manipulation. However, a thirty electric degree phase shift is still required for correct commutation, and the entire circuitry is complex. Some others directly compared the filtered average terminal voltages to obtain the required signals for commutation. For the above methods, the inclusion of low pass filters (LPFs) may solve the noise problem due to pulse width modulation (PWM) switching but will limit the speed range. In addition, some literature discussed the speed range that can be achieved under heavy load conditions.
Ji Sang Guk, a researcher at the Faculty of Electrical Engineering, has proposed a sensorless technique with a wide speed range and high efficiency using an enhanced commutation method.
This technique uses a low-pass filter with two cut-off frequencies that can be regulated for various speed operations. The higher cut-off frequency is designed for high speed to reduce phase lag, and the lower one is used for start-up to reject the noise.
From the experiments, with loads applied, he proved that the developed technique is capable of increasing the speed beyond the rated speed. For the entire speed range, the proposed technique demonstrated high efficiency even at high torque, high speed conditions.
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Jo Sep 13, 2024
Channel induction furnace (CIF) has been widely used for melting, holding and casting of metal owing to its advantages such as higher power factor, low power consumption and less stirring of molten metal because magnetic induction line follows the closed iron core.
In general, CIF is composed of induction units with iron core, inductor and channel, and a molten metal bath. The induction units are assembled in the furnace body and are separated when the furnace needs repairing. If an electric current flows through the inductor, very large induced current flows through the channel due to electromagnetic induction phenomenon. Then, the channel is heated by Joule heat from the induced current, and this heat makes the metal molten through continuous transfer into the molten metal bath. Therefore, the temperature of the channel gets higher than that of the molten metal bath during melting, and it shortens the repair cycle of the furnace due to severe erosion of lining of the channel.
To overcome such problems, Song Hak Myong, a researcher at the Department of Mechanical Engineering, analyzed the melting and flow of molten metal in the channel of CIF using COMSOL Multiphysics 5.4 and Taguchi method, and decided the design parameters of a single loop channel induction furnace (SLCIF).
The simulation result demonstrated that the proposed parameters (establishment angle of channel: 36° and cross section size of channel: 100×74mm2) can sufficiently increase the working life because of the lower maximum local superheating temperature of channel (52K) than other CIFs.
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Jo Sep 12, 2024
Fluid flow and heat transfer problems in porous media are widely applied in drying, filtration, preservation of agricultural products, fluidized bed reactors, etc.
Today, a significant amount of work has been focused on the numerical simulation of the problems involving non-equilibrium heat and mass transfer. Especially, Computational Fluid Dynamics (CFD) is used to simulate such problems.
Recently, there have been several studies on the numerical modeling of diverse problems involving heat and mass transfer including drying. In this respect, heat and mass transport equations have been proposed to model different heat and mass transfer problems based on the thermal non-equilibrium assumption.
However, few studies have been proposed on non-equilibrium heat and mass transfer models for prediction of drying process using commercial software FLUENT.
Choe Song Gun, a section head at the Faculty of Heat Engineering, has developed a numerical formulation for modeling non-equilibrium heat and mass transfer inside porous media and solved the equations by using FLUENT. To achieve the objective, he reformed the moisture transport equation and energy transport equation for the solid matrix of porous media into a user-defined scalar equation type in FLUENT. In addition, he redefined the properties of solid and some terms of these equations in consideration of the coefficients and terms of UDS equation in FLUENT.
Then, he compared the calculation results and the experimental results in order to check the validity of the reformation of the model and equations. The deviations between them are less than 3% in heat transfer and less than 8% in mass transfer. The results show that his predictions follow the expected trends with respect to temperature and velocity at inlet.
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Jo Sep 10, 2024
Wireless sensor network (WSN) is one of the most popular areas of research due to the constant improvement in the field of wireless technology and embedded system. Now, WSNs are extensively deployed in every area such as industry, civilian and military applications. These applications include monitoring, tracking, event detection and disaster management. The long term usefulness of WSN mainly relies on the lifetime of sensor nodes. The lifetime of SN mainly depends on its battery source. Since these batteries can be hardly replaceable, the improvement in the lifetime of WSNs can be achieved through the conservation of battery energy. In order to achieve this purpose, it is very important to design an energy-efficient routing protocol for efficient use of the limited energy of WSNs.
Many researchers applied the concept for clustering of sensor nodes and efficient routing to conserve the energy of sensor nodes and these techniques significantly improved the lifetime of WSN. LEACH (Low Energy Adaptive Clustering Hierarchy), the first adaptive hierarchical clustering routing protocol that considers energy saving, comprises two phases i.e. setup phase and steady phase.
But LEACH protocol has some critical drawbacks: It does not consider residual energy of a node, so a node with low energy can be selected as a cluster head. Also, due to randomized election of cluster heads, it is possible that elected cluster heads very close to each other might cause congestion in particular areas.
To solve the critical problem due to residual energy in LEACH, some protocols similar to LEACH, but considering residual energy of nodes were introduced. Nevertheless, LEACH based protocols considering residual energy of nodes can never deal with irregular election of cluster heads.
In response to the above issue, Kim Chol Man, a researcher at the Faculty of Information Science and Technology, has presented an improved cluster head election algorithm using residual energy and network address of nodes.
This algorithm is based on LEACH and considers both remaining energy and network address of nodes to revise the cluster head threshold equation. He assigned the network addresses of nodes by distributed address assignment mechanism (DAAM) of ZigBee. As he used the network address of a node in the threshold equation, the cluster type created in a new round is similar to the old cluster type. In addition, he dynamically changed the upper limit of the interval that generates random numbers to optimize the number of cluster heads.
He evaluated the performance of the proposed LEACH-NA using network Simulation platform NS-2. The simulation results demonstrate that the proposed algorithm shows better performance than the existing ones in terms of network lifetime and energy conservation, and the number of cluster heads in every round is more stable than other algorithms based on LEACH.
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Jo Sep 6, 2024
Ozone has strong oxidation ability and very strong reactivity with organic and inorganic substances. It also leaves no secondary contaminants, which makes it applicable in a variety of ways in many fields including chemical industry, medicine, light industry, agriculture, water treatment, etc.
With the expansion of application of ozone generators over a long period since its appearance, the power supply of ozone generation has been made in various types and many achievements have been made in the research for power quality improvement.
Nowadays, an ozone generator based on dielectric barrier discharge (DBD) is popular and the high-voltage and high-frequency inverter power supply is essential in power mode.
Hwang Kum Song, a researcher at the Nano Physics Engineering Institute, has proposed a method for a feedback control of the discharge current in a DBD type high-frequency and high-voltage power supply to keep the discharge intensity of the ozone generator constant and to improve the power efficiency.
He mapped the discharge current model of the DBD inverter source, which is subject to strong nonlinearity and model uncertainty, to a dynamic linearization (DL) model to design a model-free adaptive control (MFAC) algorithm, and conducted a simulation on Matlab Simulink. He designed an MFA controller with the control object equivalent to the simplest dynamic linear model.
The strong nonlinearity and model uncertainty inherent in the control objective reduce the control effectiveness of conventional control approaches (PID control or fuzzy-PID control) and complicates the application of intelligent control approaches.
However, MFAC method, which requires no model of a plant and only controls the input and output data of the system, is more effective in constant-current control of the DBD ozone generator. MFA controller based on compact form dynamic linearization is convenient for implementation because of its simple structure and low computational cost. In addition, optimizing the parameters of the model-free adaptive controller can achieve more flexible adaptation of the controller, eliminate most of the overshoot and improve the response speed.
The simulation results showed the improved effectiveness of the model-free adaptive controller compared to the PID controller or the fuzzy-PID controller.
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Jo Sep 4, 2024
Parallel manipulators are closed-loop mechanical structures presenting very good performance in terms of accuracy, rigidity and ability to move large loads. Generally, the mechanism has two platforms: one is attached to the fixed reference frame and the other performs arbitrary motions in its workspace. Some moving legs similar to those of serial robots connect the moving platform to the fixed platform. Spherical joints, revolution joints or prismatic joints connect the elements of the robot to one another.
Recently, much effort has been devoted to the kinematic and dynamic analysis of fully parallel manipulators. The dynamic analysis was usually conducted by analytical methods of classical mechanics in which projection and resolution of equations on the reference axes are written in a considerable number of cumbersome, scalar relations and the solutions are rendered by large-scale computation together with time-consuming computer codes.
Pang Thae Jin, a researcher at the Robotics Institute, has established a new method for inverse dynamic analysis of a 3-DOF parallel manipulator with three actuators using an approach based on the principle of virtual work.
The new approach is far more efficient, and it can eliminate all forces of internal joints and directly determine the time-history evolution of torques and powers required by the three actuators.
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