• Journal of Power Electronics
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• v.18 no.4
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• pp.1111-1126
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• 2018

In modern power systems, distributed generators (DGs) result in high stress on system frequency stability. Apart from the intermittent nature of DGs, most DGs do not contribute inertia or damping to systems. As a result, a new control method referred to as a virtual synchronous machine (VSM) has been proposed, which brought new characteristics to inverters such as synchronous machines (SM). DGs employing an energy storage system (ESS) provide inertia and damping through VSM control. Meanwhile, energy storage presents some physical constraints in the VSM implementation level. In this paper, a VSM mathematical model is built and analyzed. The dynamic responses of the output active power are presented when a step change in the frequency occurs. The influences of the inertia constant, damping factor and operating point on the ESS volume margins are investigated. In addition, physical constraints are proposed based on these analyses. The proposed physical constraints are simulated using PSCAD/EMTDC software and tested through RTDS experiment. Both simulation and RTDS test results verify the analysis.

• Journal of the Society of Naval Architects of Korea
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• v.53 no.1
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• pp.54-61
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• 2016

This paper defines the design constraint in consideration of the dynamic characteristics and stability in the longitudinal direction of a supercavitating vehicle. Available range of the design variables is calculated by numerical simulation and the cavity modeling of vehicle dynamics is performed first. Configuration parameters of the supercavitating vehicle to determine the vehicle dynamics and characteristics of the cavity are defined as design variables. Design constraints are supercavitation, trim velocity, stability and vehicle dynamics in transition phase. Numerical results show that in accordance with the change of the design variables, the proposed design constraints reflect the physical characteristics of the supercavitating vehicle. This research finds the design region where the constraints of supercavity and the trim velocity are satisfied, and the stability analysis refines the design results by excluding the region where the stability is not guaranteed. The stability analysis is particularly important for a vehicle with the short fin span.

• IEIE Transactions on Smart Processing and Computing
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• v.6 no.1
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• pp.1-9
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• 2017

This paper proposes to improve the performance of a strap down inertial navigation system using a foot-mounted low-cost inertial measurement unit/magnetometer by configuring an attitude and heading reference system. To track position accurately and for attitude estimations, considering different dynamic constraints, magnetic measurement and a zero velocity update technique is used. A conventional strap down method based on integrating angular rate to determine attitude will inevitably induce long-term drift, while magnetometers are subject to short-term orientation errors. To eliminate this accumulative error, and thus, use the navigation system for a long-duration mission, a hybrid configuration by integrating a miniature micro electromechanical system (MEMS)-based attitude and heading detector with the conventional navigation system is proposed in this paper. The attitude and heading detector is composed of three-axis MEMS accelerometers and three-axis MEMS magnetometers. With an absolute algorithm based on gravity and Earth's magnetic field, rather than an integral algorithm, the attitude detector can obtain an absolute attitude and heading estimation without drift errors, so it can be used to adjust the attitude and orientation of the strap down system. Finally, we verify (by both formula analysis and from test results) that the accumulative errors are effectively eliminated via this hybrid scheme.

• Transactions of the Korean Society of Machine Tool Engineers
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• v.14 no.4
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• pp.81-88
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• 2005

This paper attempts to derive the dynamic model of handling tasks in finger robot which grasps stable and manipulates a rigid object with some dexterity. Firstly, a set of differential equation describing dynamics of the manipulators and object together with geometric constraint of tight area-contacts is formulated by Lagrange's equation. Secondly, the roblems of controlling both the forces of pressing object and the rotation angle of the object under the geometric constraints are discussed. The effect of geometric constraints of area-contacts between the link's end-effector and the object is analyzed and the model based on the differential-algebraic equations is presented. In this paper, the control method for dynamic stable grasping and enhancing dexterity in manipulating things is proposed. It is illustrated by computer simulation and the experiment that the control system gives the performance improvement in the dynamic stable grasping and nimble manipulating of the dual fingers robot with soft tips.

• Journal of the Institute of Electronics Engineers of Korea SC
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• v.43 no.5 s.311
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• pp.10-18
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• 2006

We propose a new approach to compute possible acceleration boundary, so is called dynamic manipulability, for multiple robotic systems with frictional contacts between robot end-effectors and object. As the frictional contact condition which requires each contact force to lie within a friction cone is based on the nonlinear inequality formalism is not easy to handle the constraint in manipulability analysis. To include the frictional contact condition into the conventional manipulability analysis we approximate the friction cone to a pyramid which is described by linear inequality constraints. And then achievable acceleration boundaries of manipulated object are calculated conventional linear programming technique under constraints for torque capability of each robot and the approximated contact condition. With the proposed method we find some solution to which conventional approaches did not reach. Also, case studies are Presented to illustrate the correctness of the proposed approach for two robot systems of simple planar robots and PUMA560 robots.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.35 no.12
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• pp.1579-1584
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• 2011

This paper discusses an energy system that can convert wave energy into electrical energy. This wave energy generation system is movable and has 12 arms and one generator. A multibody dynamic model for this system is established by using kinematic constraints. A gear mechanism, several kinematic constraints, and force elements are included in the model. Wave forces are obtained numerically from the time domain formulation based on the Morison equation. The MSC/ADAMS program is employed to carry out dynamic analysis of the wave energy generation system. The dynamic behavior responses of this system are analyzed for design verification. According to the results of the dynamic analysis, the yaw motion is relatively stable and kinetic energy sufficient to generate electrical energy is obtained when the wave height exceeds 1m.

• 전기의세계
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• v.21 no.1
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• pp.13-16
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• 1972

Dynamic Programming is regarded as a very powerful tool for solving nonlinear optimization problem subject to a number of constraints of state and control variables, but has definite disadvantages that it requires much more computing time and consumes much more memory spaces than other technigues. In order to eliminate the above-mentioned demerits, this paper suggests a news technique called Multiple Dynamic Programming. The underlying principles are based on the concept of multiple passes that, instead of forming fin lattices in time-state plane as adopted in the conventional Dynamic Programming, the Multiple Dynamic Programming constitutes, at the first pass, coarse lattices in the feasible domain of time-state plane and determines the optimal state trajectory by the usual method of Dynamic Programming, and at the second pass again constitutes finer lattices in the narrower domain surrounded by both the upperand lower edges next to the lattice edges through which the first pass optimal trajectory passes and determines the more accurate optimal trajectory of state, and then at the third pass repeats the same processes, and so on. The suggested technique insures remarkable curtailment in amounts of computer memory spaces and conputing time, and its applicability has been demonstrated by a case study on the hydro-thermal power coordination in Korean power system.

• KDI Journal of Economic Policy
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• v.33 no.4
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• pp.1-25
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• 2011

Available evidence suggests that the average marginal propensity to consume (MPC) from the 2001 tax rebate in the US was not nearly as large as that from previous tax cuts. We examine if this phenomenon can be explained by the fact that the widespread use of credit cards has made borrowing accessible for most US households by constructing a model that simulates the dynamic effect of relaxed borrowing constraints. Our model uses Kreps-Porteus preferences which account for independent measures of relative risk aversion and the elasticity of intertemporal substitution, both of which can theoretically affect the willingness to save or spend. Our model shows that the average MPC drops substantially immediately after borrowing constraints are relaxed because few consumers have binding borrowing constraints at that time. The model also shows that consumers gradually reduce their wealth after borrowing constraints are relaxed, causing more of them to have binding constraints over time, which in turn causes the average MPC to rise gradually to a new steady state value that is slightly lower than the original value. This dynamic pattern of the MPC suggests that a greater ability to borrow with credit cards could explain the lower effectiveness of the 2001 tax rebate. In addition, the model predicts that consumers choose to hold lower amounts of liquid assets for precautionary reasons when they have a greater ability to borrow unsecured debt.

• 제어로봇시스템학회:학술대회논문집
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• 2003.10a
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• pp.215-219
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• 2003

In this paper, we generate a trajectory minimized the energy gait of a biped robot for walking a staircase using genetic algorithms and apply to the computed torque controller for the stable dynamic biped locomotion. In the saggital plane, a 6 degree of freedom biped robot that model consists of seven links is used. In order to minimize the total energy efficiency, the Real-Coded Genetic Algorithm (RCGA) is used. Operators of genetic algorithms are composed of a reproduction, crossover and mutation. In order to approximate the walking gait, the each joint angle is defined as a 4-th order polynomial of which coefficients are chromosomes. Constraints are divided into equality and inequality. Firstly, equality constraints consist of position conditions at the end of stride period and each joint angle and angular velocity condition for periodic walking. On the other hand, inequality constraints include the knee joint conditions, the zero moment point conditions for the x-direction and the tip conditions of swing leg during the period of a stride for walking a staircase.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.5 no.11
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• pp.2254-2271
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• 2011

Access control in dynamic environments needs the ability to provide more access opportunities of information to users, while also ensuring protection information from malicious users. Trust and risk are essential factors and can be combined together in access control decision-making to meet the above requirement. In this paper, we propose the combination of the trust and risk in access control to balance information accessibility and protection. Access control decision is made on the basis of trustworthiness of users and risk value of permissions. We use potential relations between users and relations between permissions in access control. Our approach not only provides more access opportunities for trustworthy users in accessing permissions, but also enforces traditional access control constraints such as Chinese Wall policy and Separation of Duty (SoD) of Role-Based Access Control (RBAC) model in an effective way.