Youn-sik Park

Robust

A robust H∞ sliding mode descriptor observer for simultaneous state and disturbance estimation of uncertain system is developed. Inspired by a singular system theory, a descriptor observer design is presented to estimate some class of output disturbances. A sliding mode scheme is used for the observer to reconstruct the input fault based on the transformed coordinate system. Simulation study illustrates the effectiveness of the proposed method.

H_{\infty}

An input shaping technique using a time-varying impulse sequence is presented to reduce the motion-induced vibration of flexible systems in a feedforward way. The decoupled modal responses for a general linear time-varying system are firstly approximated. Upon this approximation, the time-varying impulse sequences to suppress the vibrational modes are found. The reference inputs to the systems are shaped by convolving with the time-varying impulse sequence to suppress the multimode vibrations. This technique can be also applied to suppress the vibration of nonlinear time-varying systems. The performance of this method is demonstrated with two practical examples: a moving overhead crane and a two-link robot manipulator. Consequently, this study provides an input shaping technique applicable to the vibration suppression of broader classes of flexible systems.

Sliding Mode Descriptor Observer for Fault and Output Disturbance Estimation of Uncertain Systems

Vibration analysis of a piping system conveying fluid is investigated by employing the wave approach. In this paper, the inviscid fluid-dynamic forces acting on a pipe due to internal fluid flow are approximated by the plug-flow model with the slender-body theory. The straight pipe elements conveying fluid are formulated using a dynamic stiffness matrix in the frequency domain. The curved pipe sections are treated with a single curved pipe element utilizing the mixed methodology of the dynamic stiffness method and the transfer matrix method. The present approach is applied to the frequency response analysis of the 3-dimensional piping system conveying fluid which is composed of straight and curved pipe sections and is compared to the results from the finite-element formulations.

Vibration reduction in flexible systems using a time-varying impulse sequence

Recently the authors tried to find damage position only using measured frequency response functions. According to their work, it seems that the algorithm is very practical since it needs only measured frequency responses while other methods require exact analytic model. But when applying the method to a real structure, it requires lots of experiment. The authors, in this time, propose a method to reduce its experimental load by detecting damage within a substructure. This method searches damages not within an entire structure but within substructures. In addition, damage severity was treated in this paper since it is worthy to know damage severity. Optimization technique is used to estimate damage level using measured responses and damage model. Two test examples, a plate and a jointed structure, are chosen to verify the suggesting method.

Vibration analysis of a 3-dimensional piping system conveying fluid by wave approach

This paper presents a modified input shaping method to reduce the motion-induced vibration of a linear time-varying system after a rest-to-rest motion. Shaping parameters were obtained using the concept of modal-filtered impulse response. The conventional shaping method can be said just a special case of the proposed shaping method. The effectiveness of this proposing method was checked using some examples of both moderate and considerably fast time-varying systems. With a rest-to-rest motion control of a two-link flexible manipulator, this study also demonstrates that this method can be expanded to nonlinear cases.

Identification of damage on a substructure with measured frequency response functions

Integral sliding mode(ISM) based composite nonlinear feedback(CNF) tracking controller to track step signal is proposed. The proposed controller combines advantages of CNF controller like quick response without overshoot and robustness of ISM controller. Integral sliding mode is used along with CNF controller to reject disturbances and track nominal trajectory. Actuator saturation effect is considered and the stability of overall system is guaranteed with saturated actuator. Chattering is reduced by use of sigmoid function and non-linear switching gain. Effectiveness of the proposed scheme is demonstrated by simulation results.

Residual vibration reduction for a flexible structure using a modified input shaping technique

This paper focuses on overcoming the problem of tracking control in structurally flexible redundant manipulators by utilizing their self-motion capabilities. In the proposed algorithm, the self-motion is evaluated in order to nullify the dominant modal force of flexural motion that is induced by a rigid body motion.The flexure motions of manipulators, which are induced by joint motion, cause undesired inaccuracy in end-effector tracking. In-plath planning states, joint trajectories are so designed as not to excite but to damp out the flexure motions. The self-motion, inherent in redundant manipulators, can alter joint motion, influencing the flexure motion (by exciting and damping the flexure modes), while not affecting end-effector motion at all. Therefore, the self-motion can be utilized to regulate flexibility and effectively reduce the end-effector tracking error.The effectiveness and applicability of the proposed algorithm have been demonstrated through numerical simulation with three-link planar robotic manipulators possessing flexible links.

A Robust Algorithm Against Actuator Saturation using Integral Sliding Mode and Composite Nonlinear Feedback

This paper presents a method for stability analysis of a closed-loop flexible mechanism by using modal coordinates. Mode shapes of a flexible four-bar mechanism are defined as those of individual links (single-link modes). Based on these single-link modes, the flexible four-bar mechanism has time-invariant mode shapes and its governing equations of motion become decoupled, regardless of mode-crossing. Therefore the stability of the flexible mechanism can be analyzed efficiently for each mode. Floquet theory is employed to check the stability of the mechanism. The experimental study of a flexible four-bar mechanism is also presented to verify the proposed method. The experimentally-determined bending strains and critical speeds are compared with numerical results obtained from the proposed method. The experimental and analytical results show a fairly good agreement.

Self-motion utilization for reducing vibration of a structurally flexible redundant robot manipulator system

Techniques to estimate the time delay of arrival (TDOA) using the measurements of the acoustic signals by microphones have been studied in various fields such as the robot auditory system, teleconference system and speech recognition system. One common method of determining TDOA is to compute the cross correlation function. The Generalized Cross Correlation (GCC) method, which calculates the correlation function by using inverse Fourier transformation of the cross power spectral density function multiplied by the proper weighting function, was proposed by Knapp and Cater in 1976. This method analyzed the weighting functions to estimate the optimal TDOA for a single source. In this paper, we derived the cross correlation function by GCC method with PHAT weighting function for multiple sources and ascertained the relationship between the correlation value and source characteristics. Moreover, we compared the derived GCC function for two sources case with the real GCC function calculated by the actual signals and verified their similarity.

Dynamic stability analysis of a flexible four-bar mechanism and its experimental investigation

A novel headphone system with microphone arrays was proposed whose objective is to boost forward sound signal, by doing so people can communicate well with a speaker during headphone playback. The main purpose of this research is to boost the sound signal from the front, particularly for a speech signal (300 Hz~3 kHz). A delay-and-sum beamforming method is applied to an end-fire microphone array to make a proposed headphone system. Three design parameters, i.e. an aperture size, the number of microphones, and a microphone arrangement of microphone arrays are decided based on performance of beamformer with respect to each parameter. To compare the performance regarding to each design parameter, a performance measure, efficiency, was defined as ratio of two resultant sound pressures between a forward and a backward direction. A sound scattering from a head of listener is modeled by using spherical head-related transfer function (HRTF). By changing an aperture size of a microphone array, 8 cm microphone array is thought to be proper when the efficiency in speech frequency range was considered. The use of four microphones is expected as the optimum by considering both good efficiency and reducing the number of microphones. By fixing the number of microphones and an aperture size of microphone array, the efficiency was calculated in various arrangements of microphones. As a result, microphone arrangement with inter-microphone spacing of decreasing interval is selected. The design parameters such as an aperture size, the number, and an arrangement of microphones can be changed regarding to a target frequency range or a direction wanted to be boosted, and the proposed methodology can be applied similarly.