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pared with experimental data obtained in a low-speed wind tunnel over a rangę of Reynolds numbers between 5500 and 7500. Good agreement is observed between the theory and experiments. It was found that the controller is effective in attenuating the transverse vibrations of the cylinder by approximately 40% for all the flow speeds considered.

2:35

3SA3. Excitation of thin beams using asymmetric piezoelectric actuatórs. Gary P. Gibbs and Chris R. Fuller (Dept. of Mech. Eng., Virginia Tech, Blacksburg, VA 24061)

In order to analyze the excitation of a complex beamlike structure comprised of arrays of piezoelectric actuatórs (that may not be ar-ranged in symmelric pairs), it is neccssary to determine the effect of a single asymmetric actuator. Using the superposition principle, the exci-tation of the entire structure can subsequently be analyzed. New ana-lytical relations for the excitation of beams by a single piezoelectric element Iocated on the surface of the beam (asymmetric excitation) were developed. A single asymmetric piezoelectric element when ex-cited produces both flexural and extensional motion in a beam. The relations are used to study excitation of both flexural and extensionai waves in beams using a pair of actuatórs as a function of: actuator length, relative voltage magnitude and phase, and frequcncy of excita-tion. The work presented herc represents an important step because it is the analytical basis by which simultaneous active control of both flex-ural and extensional motion in flcxible structures can be studied.

tions will be discussed and mechanisms of control in terms of wave components considered. [Work supported by ONR/DARPA.]

3:20

3SA6. Active ribration control of a thin piąte using piezoelectric sensors and actuatórs. S-Y. Hong, V. V. Varadan, and V. K. Varadan (Dept. of Eng. Sci. and Mech., Res. Ctr. for the Eng. of Electron, and Acoust. Mater., Penn State Univ., 149 Hammond Bldg., University Park, PA 16802)

Using piezoelectric sensors and actuatórs, the vibrating motions of a thin piąte could be effectively controlled. Several pieces of Unimorph PZT (lead-titanate-zirconate) were optimally attached on the both sides of a clamped square piąte. A series of active vibration control experiments has been tried with different types of control algorithms. Even though the system structure is stiff enough because of the clamped edges, effective vibration control could be implemented with the large electromechanical coupling constant of PZT and well-selected positions of sensors and actuatórs. Herc, particular attention has been paid to several lower-frequency modes. To take advantage of the features of a digital system such as flexibility of the control program and inereased logie capability, a microcomputer and a data acquisition board were used to obtain a digital controller.

2:50

3SA4. Active Wbration control using shear modę piezoceramic transducers. Chia-Chi Sung, Xiaoqi Bao, Vasundara V. Varadan, and Vijay K. Varadan (Dept. of Eng. Sci. and Mech., Res. Ctr. for the Eng. of Electron, and Acoust. Mater., Penn State Univ., 140 Hammond Bldg., University Park, PA 16802)

The constant of the piezoceramics was utilized to design shear modę transducers. These shear modę transducers were verified to be pure shear stress/strain sensors/actuators both theoretically and nu-merically (ANSYS). Active torsional vibration control experiments were conducted on various phenolic and pyrex tubes by using the shear modę sensors/actuators. The free and controlled frequency responses as transfer functions were mcasured and comparcd to each other. The finite element mode-frcquency analysis was performed to evaluate the structural characteristics for the tubes with and without transducers mounted. The dynamie coupling coefficients were derived for shear modę sensors/actuators based on the finite element modę shapes.

3:35

3SA7. An experiment in State space control of a simply supported piąte excited by steady disturbnnees. S. P. Rubenstein, W. R. Saunders, G. K. Ellis, and H. H. Robcrtshaw (Ctr. for Intelligcnt Mater. Systems and Structures, Mech. Eng. Dept., Virginia Polytech. Inst. and State Univ., Blacksburg, VA 24061)

Experimental results of an active vibration control experiment on a simply supported piąte are presented. A linear quadratic Gaussian (LQG) regulator control design is implemented to reject steady distur-banccs of the plate’s fundamenta! modę. A discussion of the control law is provided. Performance of a four-mode controller is dcmonstrated for harmonie, narrow-band, and broadband point-force disturbances. Typ-ical reductions in the fundamental modę amplitudę were up to 60 dB at the disturbance center frcquencics. Results are presented in both time domain and frequency domain format The digital controller was im-plcmented on a transputer-based parał lei processing system for en-hanced computational performance. The initial sample speeds obtained with thls system were up to eight times faster than the controller hardware previously available. [Work supported by DARPA/ONR.)

3:05

3SA5. Experiments on active control of acoustic radiation from discontinuities on infinite beams. Kenneth D. Frampton and C. R. Fuller (Dept. of Mech. Eng., Virginia Polytech. Inst. and State Univ., Blacksburg, VA 24061)

Two experimenls were performed to study the active control of acoustic radiation from discontinuities on infinite beams. One cxpcri-ment studied a semi-infinite/clamped beam and the other studied an infinite beam with a blocking mass. In each case the noise input was a subsonic bending wave incident on the discontinuity. Active control was implemented using LMS algorithm driving point force shakers or piezoceramic patches with far-field microphones as error sensors. In ad-dition, an accelerometer array was employed to investigate changes in traveling and near-field waves under control conditions. Large attenu-alion in radiated sound at the error locations was noted while the beam vibrational amplitudę often inereased under control. Spatial Fourier analysis (wave number domain) of the beam vibration showed attenu-ation of supersonic wavc numbers (responsible for acoustic radiation) while under control. Results for various frequencies and test configura-

1890 J. Acoust Soc. Am., Vol. 89, No. 4, Pt 2, April 1991

3:50

3SA8. Experiments on active isolation using PVDF distributed error sensors. Sylvie Lefebvre, Catherine Guigou, and Chris R. Fuller (Dept. of Mech. Eng., Virginia Polytech. Insi. and State Univ., Blacksburg, VA 24061)

The work described in this paper is an cxperimental appraisal of active techniques for the isolation of harmonie vibrations. In particular, the use of po!yvinylidene fluoride (PVDF) strips as distributed error sensors on a receiving piąte is studied and contrastcd to the use of point sensors such as accelerometers. The experimental rig consists of an upper vibrating beam supported via natural rubber passive isolators on a rcceiving piąte with two clamped boundaries. The control approach used is a two-channel adaptive LMS algorithm implemented on a Texas Instrument TMS320C25 DSP chip. The control forces are applied through electromagnetic devioes, in parallel with the passive isolators. In generał, the results show that the PVDF sensors improve the control performance in terms of global rcduction of the response of the rccciv-ing structure, by limiting contro! spillover. The behavior of the system

121st Meeting: Acoustical Society of America 1890