John W. Shipley

Vibration isolation system using plural signals for control

An active isolation device and method reduces the transmission of vibrations from interconnected elements. The device maintains a high stiffness between interconnected elements while actively reducing the transmission of relative movements, such as vibration. In one embodiment, the device uses accelerometers to measure the vibrations experienced by each of the interconnected elements and selectively operates an actuator between the two elements to maintain the position of one of the elements. In another embodiment, plural of the devices are used cooperatively to reduce the vibrations that would otherwise be transmitted from one element to a platform, permitting simultaneous control of multiple degrees of freedom of movement.

Fatigue Damage under Wide‐Band Random Stress

Fatigue damage for a wide‐band random‐stress process is considered under the limitations of classical damage theory. A method of obtaining a lower‐bound estimate of the expected fatigue from the statistical parameters of a stress process such as the power spectral density is derived without employing extreme statistics. A method proposed by H. C. Schjeldrup for assessing fatigue damage due to a wide‐band stress process is discussed and a simplification of this method is used to obtain an upper‐bound estimate of the expected fatigue life. These two estimates are compared for some typical stress spectra.

Digital Technique for Determining One‐Third Octave Sound‐Pressure Levels with More Uniform Confidence Level

This paper describes a computer program that was written to analyze acoustical test data. It employs a fast Fourier subroutine to calculate the discrete Fourier coefficients that transform the time‐domain data to the frequency domain. In order to achieve a more equal confidence level between the upper and lower 13 octaves, the digital data record is sampled at a high rate and the fast Fourier coefficients are calculated. Next the same digital data record is sampled at a much lower rate. This gives greater low frequency resolution to the lower 13 octaves. Consideration is given to digital windowing techniques for the purpose of comparison with analogous analysis methods and their effect on spectral representation.