Papers by Author: Jung Woo Sohn

Abstract: This experimental investigation presents wear characteristics of magnetorheoloigcal (MR) fluid under boundary lubrication contact condition. Three different specimens; aluminum, copper and steel pins are prepared and wear test is performed under consideration of several operational factors such as normal load, sliding distance, and sliding speed by using pin-on-disk test equipment. Wear rate and friction coefficients of the MR fluid are experimentally evaluated with respect to the sliding ranges. Microscopic surface and roughness changes of the worn surface of pin specimens are also analyzed by using the scanning electron microscope (SEM). In addition, Energy Dispersive X-ray Spectroscopy (EDS) analysis is conducted and chemical changes are investigated.

Abstract: This paper presents temperature control of engine cooling system using a controllable magnetorheological (MR) fan clutch. An appropriate size of MR fan clutch is devised and modeled on the basis of Bingham model. Subsequently, an optimization to determine design parameters such as width of housing is undertaken by choosing the reciprocal of the controllable torque as an objective function. This has been performed using a finite element analysis. A sliding mode controller is then designed to control the angular velocity of the MR fan clutch using experimentally determined parameters. The designed controller is implemented and control performances of the MR fan clutch system are evaluated.

Abstract: In the present paper, active vibration control of cylindrical shell structure is conducted based on optimized actuator placement. Anisotropic piezoelectric actuator named as Macro Fiber Composite (MFC) is adopted for vibration control. The governing equations of motions of the cylindrical shell structure including MFC actuators are derived from Lagrange’s equation. For the verification of the proposed analytic model, numerical results of modal analysis are compared with those of experimental test results. Optimal placements of the MFC actuators are determined with Genetic Algorithm for the effective control performance. Robust controller is then designed to suppress structural vibration of the proposed smart structure and control performances are evaluated.

Abstract: In this paper, active vibration control performance of the smart hull structure with Macro-Fiber Composite (MFC) is evaluated. The governing equations of motion of the hull structure with MFC actuators are derived based on the classical Donnell-Mushtari shell theory. Subsequently, modal characteristics are investigated and compared with the results obtained from finite element analysis and experiment. The governing equations of vibration control system are then established and expressed in the state space form. Linear Quadratic Gaussian (LQG) control algorithm is designed in order to effectively and actively control the imposed vibration. The controller is experimentally realized and control performances are evaluated.

Abstract: In the present paper, the dynamic characteristics of smart hull structure are investigated for the feasibility of Macro Fiber Composite (MFC) actuator in active vibration control of smart hull structure. MFC is an advanced anisotropic piezoceramic actuator. Finite element technique was used to ensure application to practical geometry and boundary conditions of smart hull structure. Using finite element method, modal analysis and strain distribution were first conducted to investigate dynamic characteristics of smart hull structure. Based on numerical modal analysis, the locations of MFC actuators are determined to achieve maximum control authority. Then, the MFC actuators are attached to the hull structure and natural frequencies and damping coefficients are identified through experimental modal test.

Abstract: In the present paper, temperature effect on yield stress of electrorheological fluids is experimentally investigated. A rotational shear-mode type electroviscometer is designed and manufactured for the identification of Bingham characteristics of ER fluids. Optimization of ER fluids is undertaken with carrier liquid, particle and additive treatment and then four different ER fluids are prepared for the test. The field-dependent yield stress, current density and response time of optimized ER fluids are compared at various temperature conditions.

Abstract: This paper presents vibration control of a flexible smart beam structure using a new discrete-time sliding mode controller. After formulating the dynamic model in the space representation, so called the separation principle for equivalent controller is established so that the sliding mode conditions are satisfied. By doing this, undesirable chattering of the flexible structures can be attenuated in the settled phase. In order to demonstrate some benefits of the proposed methodology, an experimental realization is undertaken. Both transient and forced vibration control responses are evaluated in time domain and compared between with and without the separation principle.

Abstract: In this paper, dynamic characteristics of an end-capped hull structure with surface bonded piezoelectric actuators are studied. Finite element technique is used to ensure application to practical geometry and boundary conditions of smart hull structure. Modal analysis is conducted to investigate the dynamic characteristics of the hull structure. Piezoelectric self-sensing actuators are attached where the maximum control performance can be obtained. Active controller based on Linear Quadratic Gaussian (LQG) theory is designed to suppress vibration of smart hull structure. It is observed that closed loop damping can be improved with suitable weighting factors in the developed LQG controller.