Showing papers on "Natural frequency published in 1982"

Seismic response of end-bearing piles

Abstract: A very simple analysis of dynamic soil-pile-supported mass interaction is presented as an aid to understanding pile foundations behaviour during earthquakes. The equilibrium equations of the soil and pile movements during seismic motions are established and the response of the soil-pile system is obtained as a function of the existing frequency; for simplicity the analysis was based on Winkler's model although the same procedure can be applied using a more refined model. The effect of the supported mass is studied, the natural frequency of the system is obtained, and some practical design considerations are presented. A method for considering group effects is also presented.

Frequency synchronization and phase locking of CO2 lasers

Abstract: Employing the principle of a phase‐locked loop (PLL) we have synchronized the frequency and phase of two CO2 lasers. The laser acting as voltage controlled oscillator is tuned both by electro‐optic and piezoelectric means. A cooled photodiode serves as the phase detector. The main loop parameters, natural frequency ωn and damping factor ζ, have been measured. The small phase jitter in the error signal obtained will allow application of such an optical PLL in homodyne receivers at 10 μm.

Helicopter vibration suppression using simple pendulum absorbers on the rotor blade

Abstract: A comprehensive anaytical design procedure for the installation of simple pendulums on the blades of a helicopter rotor to suppress the root reactions is presented. A frequency response anaysis is conducted of typical rotor blades excited by a harmonic variation of spanwise airload distributions as well as a concentrated load at the tip. The results presented included the effect of pendulum tuning on the minimization of the hub reactions. It is found that a properly designed flapping pendulum attenuates the root out-of-plane force and moment whereas the optimum designed lead-lag pendulum attenuates the root in-plane reactions. For optimum pendulum tuning the parameters to be determined are the pendulum uncoupled natural frequency, the pendulum spanwise location and its mass. It is found that the optimum pendulum frequency is in the vicinity of the excitation frequency. For the optimum pendulum a parametric study is conducted. The parameters varied include prepitch, pretwist, precone and pendulum hinge offset.

Optimal Grillage Layout for Given Natural Frequency

Abstract: The problem of minimizing the volume of structural material of an elastic grillage subject to a prescribed value or the fundamental frequency of free transverse vibrations, is considered. The domain and the boundary conditions are given, and the grillage is equipped with a specified, distributed non-structural mass, which is large in comparison with the structural mass. It is the objective to determine the optimal layout of the grillage, and to optimize the beams, which are of given depth and varying width. It is shown that the optimal grillage layout is the same as that found earlier for three different types of design problems, namely plastic design for given collapse load, elastic design for given permissible stress, and elastic design for given compliance. An an example, a simply supported, square grillage with uniformly distributed mass loading is optimized, and it is shown that the amount of structural material of the optimal grillage is significantly smaller than that to be used for a uniform plate with the same fundamental natural frequency of transverse vibrations.

Axisymmetric Vibrations of a Vessel with Variable Thickness

Abstract: In this paper, the authors investigate the axisymmetric vibrations of a vessel with variable thickness composed of a cylindrical shell and two circular plates as its lids. They obtain the Lagrangian of the vessel expressed by quadratic forms of unknown boundary values and obtain the frequency equations from the minimum condition of the Lagrangian. Effects of the variations of thickness of circular plates and a cylindrical shell, of the length of a cylindrical shell etc.upon natural frequencies and mode shapes are clarified in discussions of numerical results.

Channel changeover system of frequency synthesizer

Abstract: PURPOSE:To shorten a channel changeover time and to achieve an accurate high- speed channel changeover by increasing the natural frequency of a PLL circuit on the start of the channel changeover and by making the frequency stationary at the point in time when a rough adjustment is made. CONSTITUTION:Between a reference frequency generator 1 and a phase comparator 5, a variable frequency divider 8 is provided, and its frequency division number is set in a changeover controlling circuit 9. During a channel chageover, this controlling circuit 9 switches the frequency division ratios of the frequency divider 8 and a variable frequency divider 4, inputting the output of a VCO3, and the constant of a loop filter 6. When the output frequency of the VCO3 is varied by varying the frequency divison ratio of the frequency divider 4 constituting a PLL circuit, a reference comparison frequency is mltiplied by (n) (n; integer) only in a constant time after the start of the switching, and at the same time, the frequency division number of the frequency divider 8 is equalized to a value obtained by dividing the frequency division number in a stationary state where the switching is copleted by (n). Further, the constant of the filter 6 is adjusted to a value matching with a nearly (n) fold reference frequency to change channels of a synthesizer accurately in a short time.

Method of converting fluid energy

Abstract: PURPOSE:To provide an energy converter whose constitution is simple and which is unlikely to be damaged and produces a constant output, by placing a vibration system in a flow so that the system vibrates in parallel with the flow and by changing the energy of the flow into the reciprocation energy of the vibration system. CONSTITUTION:A pressure reception plate 1, which receives the pressure of a fluid, is supported on one end of a strut 2 and placed in the fluid. The other end of the strut 2 is secured. The pressure reception plate 1 and the strut 2 constitutes a vibration system. The natural frequency of the system can be selected by appropriately setting the length and thickness of the strut 2. The the vibration system performs reciprocation at the period determined by the natural frequency, centering a position tilted to the downstream side. For that reason, an appropriate device is attached to an optional point of the vibration system, for example, a point 3 to take out the reciprocation energy of the vibration system. Since the vibration system is simply composed of the pressure reception plate 1 and the strut 2, the system is unlikely to be damaged. In addition, a constant output is obtained because of the natural frequency.

Pendulum-type dynamic vibration absorber

Abstract: PURPOSE:To greatly reduce the frictional force which acts to a weight and to simplify the mechanism of a pendulum-type dyamic vibration absorber, by hanging the weight with a plurality of hangers at or near the top of a structural body and providing a damper which acts in the direction of vibration to be controlled. CONSTITUTION:Hangers 3 are attached to a support frame 1 in such a manner that the upper attached points of the hangers are spread on a plane perpendicular to the direction of that vibration of a structural body, which is to be controlled. As a result, the motion of a weight 2 is provided with a directionality. The natural frequency of a pendulum-type dynamic vibration absorber is set by adjusting the length of the hangers 3. The damping coefficient of the absorber is set by adjusting that of an oil damper 5 or by adjusting the lever ratio of a connecting rod 4. Various properties are thus adjusted to achieve optimal vibratory characteristic values against the vibratory characteristic values of the structural body whose vibration is to be restrained, to provide the dynamic absorber with a desired damping force. Since the frictional force in the absorber and its scale are small, the absorber can be used as a vibration restrainer for a main tower being built.

Natural oscillations of suspension bridges

Abstract: Natural modes and frequencies of a simple span suspension bridge with straight backstays are investigated, theoretically and experimentally. Fourier series representations of cable displacements are used in conjunction with the lagrange method to represent the system with a small number of degrees of freedom. Bridge system matrices are derived which take account of cable, gravity, bending, and torsional stiffnesses. Natural modes and frequencies are obtained as the eigensolutions of small order system matrices. A numerical example is given from which approximate frequency formulas are derived. Experimental results in the form of frequency response curves, are presented for a laboratory model. Agreement between theory and test is within 5 per cent or better (a).

Audio device for vehicle

Abstract: PURPOSE:To prevent a peak and dip from being generated in a reproduction frequency characteristics, by attaching a viscoelastic member for suppressing a natural oscillation of a damper to the damper to prevent a diaphragm from vibrating with the natural frequency of a damper beam part. CONSTITUTION:A viscoelastic member 21 for suppressing a natural oscillation of a damper 6 is attached to the surface part of the damper 6. If a sound signal is inputted in a coil 13, a magnet side 4 and an exciting plate side 3 displace and oscillate relatively through the damper 6, and a diaphragm 16a is excited with the oscillation of the exciting plate 3, etc., to reproduce a low sound range. The beam of the damper 6 tends to oscillate due to its natural frequency. However, the oscillation is absorbed and damped by the viscoelasticity of the viscoelastic member 21 to suppress the generation of oscillation to the outside. As a result, a peak and dip of a frequency part corresponding to the natural frequency of the beam is almost eliminated in a reproduction frequency characteristics.

Modal vector estimation for closely-spaced-frequency modes

Abstract: Techniques for obtaining improved modal vector estimates for systems with closely spaced frequency modes are discussed. In describing the dynamical behavior of a complex structure modal parameters are often analyzed: undamped natural frequency, mode shape, modal mass, modal stiffness and modal damping. From both an analytical standpoint and an experimental standpoint, identification of modal parameters is more difficult if the system has repeated frequencies or even closely spaced frequencies. The more complex the structure, the more likely it is to have closely spaced frequencies. This makes it difficult to determine valid mode shapes using single shaker test methods. By employing band selectable analysis (zoom) techniques and by employing Kennedy-Pancu circle fitting or some multiple degree of freedom (MDOF) curve fit procedure, the usefulness of the single shaker approach can be extended.

Rotary support for rolls of convoluted webs and means for damping its natural frequency oscillations

Abstract: A roll of convoluted paper web rests on two parallel supporting rollers having hollow housings for coaxial dynamic damping systems. The damping systems have cylindrical masses with trunnions which are surrounded by rubber rings fitting into sleeves which are snugly inserted into the hollow housings of the respective supporting rollers. The rings maintain the peripheral surfaces of the masses out of contact with the internal surfaces of the corresponding sleeves. The natural oscillation frequency of each dynamic damping system is or can be attuned to the natural oscillation frequency of the corresponding housing prior to assembly of the housings with the associated damping systems. The damping systems ensure that the peripheral surfaces of the housings remain in at least substantially uninterrupted contact with the outermost convolution of the roll of convoluted paper web. If the web is to be collected by a core within the roll, one of the housings is driven to move the web lengthwise from a source of supply toward the roll.

Abnormal Vibration Problems in Large Turbine-Driven Generators and Their Solutions

Abstract: Some unusual causes of abnormal vibration in large turbine-driven generators, are presented along with extensive test data on a 416 MVA generator equipped with many internal thermocouples and vibration transducers. The behavior of this and other large generators after a period of operation appeared to indicate buckling of the stator laminations is occurring which lowers the natural frequency of the core structure in the elliptical vibration mode. Moreover, experience has also shown that internal temperatures within the core structure sometimes affect the natural core frequency.

Technical Notes: Natural Frequency of Rotating Beams Using Non-Rotating Modes

Abstract: A Lagrangian approach is formulated for predicting the rotating natural frequencies of a beam from the nonrotating modes and frequencies. Using the first two nonrotating mode shapes in one case and the first four such modes in the other case the frequencies of the rotating modes are calculated for a short tapered beam and a typical helicopter blade, respectively. In each case the beam is represented by lumped parameters. The number of mass points representing the beam and the accuracy of the calculated slopes of the nonrotating mode shapes at those points are both shown to affect the accuracy of the resulting frequencies, but the number of stations is shown to be more important.

Vibration-proof device

Abstract: PURPOSE:To effectually suppress the vibration of a long rod, by attaching a vibration system made of individual members of appropriate natural frequency and damping properties, to the natural frequency vibration loop part of the long rod to reduce the harmful natural frequency vibration loop amplitude of the rod. CONSTITUTION:A vibration system made of a damper C, a mass M and a spring K and having a natural frequency as a single member, which is by 20-50% lower than the harmful natural frequency of a long rod, is provided near the natural vibration loop of the rod. A spring 2 equivalent to the spring K is provided concentrically to a vertical long rod 1, for example, so that the lower end of the spring 2 is in contact with the rod 1. A weight ring 3 equivalent to the mass M is provided concentrically to the rod 1 so that an appropriate clearance is set between the ring and the rod and the ring is secured on the upper end of the spring. A sliding ring 4 is provided on the former ring 3 concentrically to the rod 1 so that an appropriate clearance and an adequate frictional force are set between the ring 4 and the rod. This results in effectually suppressing the vibration of the long rod such as a metal rod, the damping by the material of which is low.

Measuring method for water-cement ratio of concrete which has not set yet

Abstract: PURPOSE:To make the gradient of amplitude displacement larger with respect to a change in a water-cement ratio by vibrating sensing plates at the driving frequency coinciding with the natural frequency of concrete which has not ser yet CONSTITUTION:The output of an oscillator 8 which oscillates the frequency equal to the natural frequency of concrete which has not set yet is set at the output corresponding to the consistency of the concrete by an ammeter 10 at an amplifier 9, and drives an electromagnetic driving source 6 On the other hand, sensing plates 5 of a biconvex lens shape of which the top ends of legs 4 are fixed to a pair of leaf springs 3 mounted to both sides of a body 1 of a measuring machine and which are fixed to the leading ends of the legs 4 are excited by the source 6 and vibrate to the right and left The signals from a pickup 7 are inputted via an amplifier 11 to an indicator 12, by which the rates of vibration displacements are displayed Thereby, the gradient of amplitude displacements is increased and the water-cement ratio is measured accurately

On the Optimal Location of Vibration Supports.

Abstract: : The problem of optimal positioning of vibration supports to raise the fundamental natural frequency of a system is studied It is established that for the optimal locations criterion the corresponding lowest antiresonant frequency is a maximum A numerical example illustrates this criterion (Author)

Vibroacoustic Modeling for Space Shuttle Orbiter Thermal Protection System

Abstract: A ceramic tile has been developed as part of the thermal protection system of the Space Shuttle orbiter. These tiles are individually attached to the panels of the orbiter through a flexible pad. Closed-form and numerical linear random vibration models were formulated to predict tile dynamic stresses due to the broadband acoustic field on the outer surface and the base excitation on the bonded surface. Analytical results were compared to a full-scale random test. The analytical models were then used to predict dynamic stresses on selected tiles. Conditions were established as to the accuracy of the closed-form techniques. Nomenclature As = strain isolation pad (SIP) surface area A ( = tile exposed surface area c = linear viscous damping coefficient of SIP dB = decibels dco = frequency differential, rad/s fj = /th frequency, Hz fn = natural frequency of tile, Hz F (t) = acoustic force acting on tile g = acceleration of gravity H(fi) = transfer function between tile acceleration and panal acceleration for the /th one-third octave band //;(/,) = transfer function between tile velocity and acoustic pressure for the /th one-third octave band H2 (ff) = transfer function between tile displacement and

Running controlling method for operation of axial void type rotary electric machine

Abstract: PURPOSE:To avoid the number of critical rotations and to secure stable rotation by switching power frequency at several steps to avoid the natural frequency of a rotor film. CONSTITUTION:The number of rotations of a motor 10 is always supervised by a detector 15 for rotation and the detected frequency and previously set switching frequency are compared by a comparator 14. When the detected frequency exceeds the switching frequency, the pattern output in a power source pattern generator 13 is changed and the voltage and frequency of an inverter power source 12 is set to a new value increased by one step. In this way, the resonance frequency of a rotor disc can previously be avoided and the breakdown of a motor rotor at a resonance point can completely be prevented.

Analysis of Vibration by Component Mode Synthesis Method : Part.2 Forced Vibration, (I)

Abstract: A method is presented to analyze the forced vibration of a complex mechanical structure by using the natural modes of its components. The structure is divided into some components. All components are classified into master components and branch ones. The natural modes of each component are determined separately by the finite element method. The natural modes of all components are synthesized to form generalized system coordinates. The equation of motion under these system coordinates is solved to know the natural frequencies and the natural modes under these system coordinates. Using these natural modes, the response of the forced vibration is solved by the technique of the modal analysis. The forced vibration of three specimens is analyzed by the present method. On the other hand, the forced vibrations of these specimens are measured experimentally under the harmonic exciting force to get the compliances. The calculative results are compared with the experimental ones to check up on the accuracy of the calculation and availability of the proposed method.