Showing papers on "Natural frequency published in 1980"

Rotating body sympathetic vibration absorber

Abstract: In a vibration absorber for resonance vibrations of rotating bodies, in particular rail wheels, comprising several oscillatory plates which establish a damping mass; intermediate layers of damping material separating the plates from each other; a fastener connecting the plates with the rotating body at at least one point and an intermediate layer of damping material coupling the plates over a large remaining area to the rotating body the geometric dimensions of the plates and the intermediate layers and the Shore hardness of the damping material of the intermediate layers is selected such that all plates oscillate at a natural frequency of the rotating body to be damped, but with the amplitudes and phases of the vibrations of the individual plates differing in such a manner, with respect to one another, that the damping material is as strongly distorted as possible.

Means for damping mechanical torsional oscillations of an electric a.c. generator

Abstract: For damping torsional oscillations of an electric a.c. generator, a speed-sensing member, for example a tachometer-generator, is connected to the a.c. generator. The speed-dependent signal generated by the speed-sensing member is supplied to a band-pass filter which is adapted to the natural frequency of the torsional oscillations. The output signal of the band pass filter is supplied directly to the control angle determining means of a converter connected to the generator. In this way, the power of the converter is modulated in such a way that the oscillations are damped.

Fixed position variable frequency pendular-type vibration absorber

Abstract: A variable frequency vibration absorber adapted to be fixedly mounted in a fixed vibration prone system to coact with the system principal vibration excitation source to control system vibration. The vibration absorber is bifilar in construction and the natural frequency thereof is varied in proportion to the frequency of the vibration being generated by the principal system excitation source.

Ultra sensitive liquid level detector and method

Abstract: A highly sensitive liquid level detection system. It employs a piezoelectric transducer that has a natural frequency of vibration which effectively varies depending upon the amount that the transducer is immersed in a liquid. An emf is applied to the transducer to cause it to vibrate at the effective natural frequency, and the exact frequency is counted which count indicates the amount of immersion with a high degree of accuracy. A printer may be employed to periodically record the frequency count.

Lower Mode Response of Circular Cylinders in Cross-Flow

Abstract: Transverse amplitude responses of a circular cylinder in cross-flow were determined as a function of reduced velocities for a variety of spring constants and damping coefficients. Maxima were found at reduced velocities of 5 and 16, and were of comparable amplitude. The first resonance, designated the “fundamental mode,” was due to normal vortex street excitation of the spring-mass system. The second resonance, designated the “lower mode,” occurred when the natural frequency was approximately one-third of the normal vortex shedding frequency. By assuming that the driving force was sinusoidal, it was possible to evaluate the lift coefficients at resonance. Lift coefficients for the lower mode behaved similarly with amplitude ratio but were an order of magnitude lower than lift coefficients for the fundamental mode. A mechanism was used to oscillate the cylinder transversely at prescribed frequencies and amplitudes. Dominant wake frequencies were determined from a frequency analysis of the hot-wire signal for a range of velocities and a fixed frequency of oscillation. It was found that synchronization of the shedding frequency to the forcing frequency did not take place for the lower mode. The familiar “lock-in” region, or frequency synchronization over finite bandwidth, was observed for the fundamental mode only. Since the frequency associated with normal vortex shedding was not suppressed when oscillations took place in the lower mode, it would seem that a low frequency vortex street had not replaced the normal one. It is likely, then, that the spring-mounted cylinder responded subharmonically to the exciting force resulting from vortex shedding. In this regard, however, it was curious that subharmonic response was not found at a frequency ratio of 0.5 as it was at 0.33. A conceptual model, which incorporated features of both the low frequency vortex street and subharmonic response, was developed which accounted for lower mode response at a frequency ratio of 0.33 as well as the lack of response at 0.5.

The estimation of natural frequencies and damping ratios of offshore structures

Abstract: This paper focuses on the estimation of natural frequencies and modal damping ratios from measured response spectra, with particular emphasis on the dynamic response of offshore structures to wind and wave excitation. At present, estimates of natural frequencies and damping ratios are computed from the location and half-power bandwidths of resonant peaks in a structure's ambient response power spectrum. While reliable natural frequency estimates are typically obtained in this manner, half-power bandwidth damping estimates are shown to be highly sensitive to the method employed in estimating the response spectrum. The lack of confidence bounds on natural frequency and damping estimates further restricts the utility of the estimates. An alternative method is developed based on a powerful method of spectral estimation known as the Maximum Entropy Method (MEM). The resulting technique yields estimates of natural frequencies and modal damping ratios as well as approximate statistics on the reliability of the estimates. Performance of this new method is explored through extensive Monte Carlo simulation of one and two degree-of-freedom systems. Conventional estimates are also simulated for comparison with the MEM parameter estimator. The use of the MEM parameter estimator is further illustrated with ambient response data from Shell Oil's South Pass 62C platform. The MEM parameter estimates show excellent agreement with natural frequency and damping estimates obtained during recent tests conducted using forced excitation.

Stabilizing structures against oscillation

Abstract: A device for stabilizing structures such as chimneys and like stacks against transverse oscillation, comprising a mass connected to the structure by a pendulum suspension so as to oscillate in a direction transverse to the axis of the structure. The pendulum suspension includes a damper. The oscillation frequency of the mass/pendulum/damper assembly is matched to the natural frequency of the structure's vibrations, whereby the latter are attenuated. The mass may be in the form of a ring, suspended from the top of the structure by at least three pendulum/damper units spaced around the periphery. The units may also include springs or other devices to modify the natural frequency of oscillation of the device. In one version of the invention the mass comprises a first ring suspended from the structure, and a second ring similarly suspended from the first ring.

Fixed position, fixed frequency pendular-type vibration absorber with frequency linearization

Abstract: A fixed frequency vibration absorber adapted to be fixedly mounted in a fixed vibration prone system. The vibration absorber is of the pendular-type with two dynamic masses suspended in pendular fashion from a base member, and with at least one coil spring acting upon the masses to establish and linearize the vibration absorber natural frequency.

Vibrating beam pressure sensor

Abstract: A sensor providing an output indicating pressure including means to excite a vibrating beam at its natural frequency, and means to load the beam in response to pressure to alter the natural frequency of the beam. The frequency output is sensed by a capacitor pick off mounted on the same side of the beam as the means to excite the beam. Changes in the output frequency provides an indication of the pressure being measured.

The tank wall effect on internal waves due to a transient vertical force moving at fixed depth in a density-stratified fluid

Abstract: The fluid is incompressible, inviscid and non-diffusive. It has a uniform Brunt–Vaisala frequency, N, and is of constant depth, D. A body or wing moves horizontally through the fluid at velocity U in a straight line, exerting a vertical force during a given time interval. The force is constant, or oscillatory with frequency σ. The vertical average of the strain rate in a thin surface layer is calculated for a network of points behind the body.The linearized analysis is first applied with tank walls, then modified for remote walls and a vertical force of long duration.For moderately high velocity and forcing frequency (U/ND = 5, σ/N≅ 4−16) the recurring internal wave pattern just behind the body is well established in one cycle of the oscillatory force. A tank width one or two times the depth gives good agreement between tank and no-wall calculations for the chosen examples.For a stationary wing (U/ND = 0) in a cubic tank with forcing frequency one-half the natural frequency (σ/N = ½) the strain rates after one cycle are 103 times greater than for the moving wing case. After five cycles the magnitudes are twenty times larger than after one cycle. Presumably these large increases are due to the continuous and efficient feeding of energy into a small fluid volume which occurs for the stationary wing. No-wall calculations for many cycles give amplitudes roughly one-half those for five cycles in the tank, showing the effect of escaping energy.The relation of these developments to stationary phase analysis and preferred directions is discussed.

Damping measurements on an offshore platform

Abstract: Ambient and forced vibration tests were recently conducted on an offshore steel template platform in the Ekofisk complex of the North Sea. Valuable information about the dynamic characteristics of the structure was obtained. This paper focuses on the measurement and analysis of damping data for the platform. Modal damping values are very important in the dynamic analysis of offshore structures. Correct values of modal damping need to be utilized for the dynamic analysis to accurately predict the response of the structure. Little experimentally verified information of the modal damping characteristics of the fundamental and higher order modes of offshore structures is currently available. Damping values were calculated for the fundamental and higher order jacket modes which included natural frequencies up to approximately 5 Hz. Damping values are compared for forced and ambient vibration data, for bandwidth and response techniques, and for different excitation levels. The general results indicated that the fundamental modes important in fatigue had damping values of 1 to 3 percent of critical and the higher order modes had damping values of 2 to 3 percent of critical. Emphasis is placed on the apparent merits and limitations of the data presented. Response comparisons are also presented using an analytic model of the platform with various values of critical damping.

Differential weighing system providing improved signal to noise ratio

Abstract: A differential weighing system is provided which includes a primary suspension having an associated weigh pan for receiving a commodity to be weighed, a secondary suspension having a natural frequency matched to that of the primary suspension, and an electrical displacement transducer for producing an output in accordance with the relative displacement between the two suspensions. Damping to ground is provided for each suspension in combination with damping of the relative movement between the suspensions. The two inputs of a differential amplifier are connected to the outputs of a pair of sensing coils of the displacement transducer. A low pass filter having a cutoff frequency at or near the natural frequency of the suspensions is connected to the output of the differential amplifier.

Rotary knife holder with means for damping its natural frequency oscillations

Abstract: A rotary knife holder which is used in a transverse cutter for running paper webs or the like has a tubular carrier with an axial bore for a dynamic damping system serving to damp the natural frequency oscillations of the carrier. The damping system has a cylindrical mass whose end portions are surrounded by elastic annuli installed in a sleeve which is fixedly installed in the axial bore of the carrier. The natural frequency of the damping system is attuned to the natural frequency of the carrier. If the carrier is journalled at both ends, the damping system is installed midway between the ends of the carrier.

V/STOL Equivalent Systems Analysis.

Abstract: : A fixed base manned V/STOL handling qualities simulation was performed to answer questions concerning: (a) the pilot's frequency range of interest, (b) the maximum allowable mismatch between high order systems and their low order equivalents, (c) acceptability of high order appearing responses and (d) the piloting effects of phase lag at the natural frequency versus time delay. Various high or low frequency terms were added to low order attitude systems. Pilot ratings and comments showed that the pilot's frequency range of interest was .5-4.0 rad/sec; the amount of mismatch allowed was dependent on the type of high order dynamics added and the match frequency range; pilots did not accept high order appearing dynamics and; time delay is a better correlating parameter than phase lag at the configuration natural frequency. Equivalent systems analysis established criteria for the different flying qualities levels in terms of equivalent parameters.

Spring-mass accelerometer using piezoelectric tuning fork assembly - consisting of four piezoelectric elements of identical characteristics joined together in pairs by carrier

Abstract: A spring-mass accelerometer is such that the displacement of the mass against the force of the spring is a measure of the applied acceleration and is converted into an electrical signal. It reduces the incidence of spurious voltages indicating accelerations not actually existing which can arise in piezoelectric types. Four piezoelectric elements of identical mechanical characteristics are joined together by a carrier (1) in pairs (2, 3) to form a tuning fork like arrangement electrically connected. An a.c. voltage of frequency corresp. to the mechanical natural frequency of the elements is connected to the first pair (2). The second pair (3) produces a voltage whose phase angle w.r.t. the input voltage is a measure of the acceleration. Because a.c. voltage stimulation is used there is no rest condition in which the null voltages can occur.

Analysis of the air-suspension stem-vibration strawberry harvesting concept.

Abstract: ANALYTICAL models of the forced vibration of fruit-stem systems were developed. The nonlinear equa-tions of motion were numerically solved to obtain the dynamic behavior of the system. High stem tensile forces were predicted for forcing frequencies near the tilting mode natural frequency and approximately twice the tilting mode natural frequency. Natural frequencies and mode shapes for the fruit-stem system were obtained by a linear analysis. Relation-ships between the natural frequencies and system param-eters were developed.

Method of absorbing vibration of machine foundation or the like

Abstract: PURPOSE:To maintain the static or dynamic balance of a system as a whole and absorb the vibration of a rigid body, by providing two dynamic vibration absorbers which are different in various constants and lacated in different positions on the rigid body. CONSTITUTION:When a vibrating rigid body 10 such as a machine foundation vibrates on a spring 13, dynamic vibration absorbers 11, 12 are provided in two positions to absorb the vibration. The dynamic vibration absorbers 11, 12 are conventionally composed of mass parts 14, 15, springs 16, 17 and dampers 18, 19 and different in various constants such as the mass, spring stiffness and viscosity. The natural frequency of each dynamic vibration absorber is set near the frequency of a stationary vibration source. The dynamic vibration absorbers 11, 12 are mounted in such positions on the right and left or rear and front or the like of a rotary vibrating machine that a foundation system including the machine is statically balanced. This results in enabling the enlargement of the absorbed vibration frequency range, absorption of higher mode of vibration, etc.

Vehicle collision sensor with spring-mounted maps - accommodated in housing contg. damping fluid and detecting contact of mass against sidewall

Abstract: A sensor, e.g. or collision sensor, producing an output signal when the corresp. vehicle protection device is subjected to a certain acceleration time profile has a mass mounted on springs in a housing which operates a contact on touching a housing side wall. It is simply constructed and adjusted and produces reliable indication when required. This is ensured by filling the chamber contg. the mass with a damping fluid giving a damping ratio between 0.2 and 0.7. The natural frequency of the mass suspended by a sprung rod is 25 H2 or more. The max. deflection of the mass before contact is made with the side-walls may be varied by varying the height of the mass w.r.t. its sloping housing contact surface.