Yun-Ho Shin

Three Degrees of Freedom Active Control of Pneumatic Vibration Isolation Table by Pneumatic and Time Delay Control Technique

Based on previous feasibility study on one degree of freedom (1DOF) pneumatic active control of pneumatic springs, this paper presents procedures and results of a more realistic 3DOF active control of a pneumatic vibration isolation table. The 3DOF motion of the pneumatic table, consisting of heaving, rolling, and pitching, is controlled directly by adjusting air pressure in four pneumatic cylinders in a dynamic manner with pneumatic valves, without any external actuator such as an electromagnet or voice coil. The time delay control, which is a software chosen in this study, together with the hardware, i.e., the pneumatic actuator, is shown to be very powerful in enhancing the performance of vibration isolation for ground excitation as well as in settling time reduction for payload excitation through simulations and measurements on the 3DOF motion control system. New key results found in the experimental approach are that the pneumatic actuator shows a dynamic behavior of a second-order system, instead of a first-order system, which has been used in existing literatures so far, and that just feed-forward control of the pneumatic actuator by the second-order model can compensate for the inherently slow response characteristics of the pneumatic actuator very successfully. Effectiveness of the proposed active pneumatic control technique in the multi-input and multi-output system is shown via singular value decomposition analysis on the transmissibility matrix. Promising future of the proposed control and performance analysis technique is further discussed based on the results in the case of payload excitations as well.

A Study on Response Analysis of 6-DOF Pneumatic Vibration Isolation Table Loaded by Transient Movements of Carriage on It

As environmental vibration requirements on precision equipments get more stringent, use of pneumatic vibration isolators becomes more crucial and, hence, their dynamic performance needs to be further improved. Dynamic behavior of those pneumatic vibration Isolation tables is very important to both manufacturer and customer as performance specifications. Together with conventional transmissibility, transient response characteristics are another critical performance index especially when movements of components, e.g., x-y tables, of the precision equipments are very dynamic. In this paper, analysis on transient response of a pneumatic vibration isolation table loaded by a mass moving on it is presented. This is a conventional dynamics problem on a rigid body with 6 degree of freedom and a mass with another degree of freedom. How to obtain transient responses of the isolation table is described when the movements of the mass are prescribed relative to the table.

Performance Enhancement of Pneumatic Vibration Isolator in Low Frequency by Time Delay Control

As environmental vibration requirements on precision equipment become more stringent, use of pneumatic isolators has become more popular and their performance is subsequently required to be further improved. Performance of passive pneumatic isolators is related to various design parameters in a complicated manner and, hence, is very limited especially in low frequency range by chamber volume. In this study, transmissibility behavior of the pneumatic isolators depending on frequency and dynamic amplitude are presented. Then, an active control technique, time delay control, which is adequate for a low frequency nonlinear system, is applied. A procedure of applying the time delay control law to a pneumatic isolator is presented and it`s effectiveness in the transmissibility performance is shown. Comparison between passive and active pneumatic isolators is made based on simulation.

A Method to Determine Optimum Viscoelastic Layer Thickness of Sandwich Plate for Maximum Modal Damping

Thickness of damping layer in sandwich plate needs to be optimized in order to make modal loss factor of the sandwich plate maximum. Since previous studies were interested in noise reductions over high frequency range, the modal properties were derived based on simply supported boundaries. This conventional formula is approximately applicable to other boundary conditions over high frequency range only. The purpose of this study is to propose a method to determine optimum damping layer thickness of sandwich plate for maximum modal damping in low frequency range when the boundary condition is not a simple support. The conventional RKU equation based on simply supported boundary is modified to reflect other boundary conditions and the modified RKU equation is subsequently applied to determine the optimum damping layer thickness for arbitrary conditions. In order to reflect frequency-dependent characteristics of elastic modulus of the damping layer, an iteration method is proposed in determining the modal properties. Test results on sandwich plates for optimum damping layer thickness are compared with predictions by the proposed method and conventional method.

Performance Enhancement of Pneumatic Vibration Isolation Tables in Low Frequency by Active Control

As environmental vibration requirements on precision equipment become more stringent, use of pneumatic isolators has become more popular and their performance is subsequently required to be further improved. Dynamic performance of passive pneumatic isolators is related to various design parameters in a complicated manner and, hence, is very limited especially in low frequency range by volume of chambers. In this study, an active control technique, so called time delay control which is considered to be adequate for a low frequency or nonlinear system, is applied to a single chamber pneumatic isolator. The procedure of applying the tine delay control law to the pneumatic isolator is presented and its effectiveness in enhancement of transmissibility performance is shown based on simulation and experiment. Comparison between passive and active pneumatic isolators is also presented.

Destructive frequency of oblate spheroidal air-balloon for suppression of propeller cavitation induced hull excitation

The air-balloon can effectively neutralize hull excitations induced by the propeller cavitation. For the design, it is essential to derive the destructive frequency of an oblate spheroidal air-bubble, which is elaborated on in this paper. Beginning with the exact modal-series solution proposed by Yeh [Ann. Phys. 468, 53-61 (1964)], an approximated form of the scattered pressure is set up by assuming that the acoustic wavelength is much larger than the size of the balloon in the low frequency ranges. An algebraic formula for the destructive frequency can then be written as a function of the resonance frequency and a spatial variable. It is well known that the resonance frequency of a deformed bubble is higher than that of an ideal spherical one with the same volume. In addition to this, the current investigation puts an emphasis on the fact that asphericity induces a more severe shift of the destructive frequency than the resonance frequency, and that its effect needs to be reflected in the balloon design.

Improvement of dual chamber pneumatic vibration isolators by using input-output linearization and Time Delay Control

Existing controls for the dual chamber pneumatic vibration isolator (DC-PVI) have the following problems: (1) unable to suppress both seismic vibration and direct disturbance (or payload disturbance) simultaneously; (2) complex and of high-order. In this paper, we have applied input-output linearization to the DC-PVI to derive a reduced-order model that enables the followings: (i) a reduced-order control design; (ii) simultaneous suppression with an accelerometer only. We have employed Time Delay Control (TDC), well-known for its simplicity and robustness to uncertainties and disturbance. Based on the reduced-order model and its corresponding TDC design, an active control scheme has been derived that requires only an accelerometer for its sensor. This scheme was experimented with a DC-PVI, which successfully suppresses seismic vibration and direct disturbance both separately and simultaneously. Faced with seismic vibration, the transmissibility of the DC-PVI with TDC has virtually no resonance peak at low frequency; under direct disturbance, the DC-PVI with TDC achieves a 65 percent reduction in settling time of that without TDC.

Consideration of Static-Strain-Dependent Dynamic Complex Modulus in Dynamic Stiffness Calculation of Mount/Bushing by Commercial Finite Element Codes

Little attention has been paid to static-strain-dependence of dynamic complex modulus of viscolelastic materials in computational analysisso far. Current commercial Finite Element Method (FEM) codes do not take such characteristics into consideration in constitutive equations of viscoelastic materials. Recent experimental observations that static-strain-dependence of dynamic complex modulus of viscolelastic materials, especially filled rubbers, are significant, however, require that solutions somehow are necessary. In this study, a simple technique of using a commercial FEM code, ABAQUS, is introduced, which seems to be far more cost/time saving than development of a new software with such capabilities. A static-strain-dependent correction factor is used to reflect the influence of static-strains in Merman model, which is currently the base of the ABAQUS. The proposed technique is applied to viscoelastic components of rather complicated shape to predict the dynamic stiffness under static-strain and the predictions are compared with experimental results.