Active isolation of vibration with adaptive structures

The problem of actively isolating the periodic vibrations of a rigid machine mounted on a supporting flexible structure is usually approached by applying the active inputs in parallel or series with the passive inputs. This has a number of disadvantages which are related to the development of a high power, compact yet stiff/active isolation unit. In this experimental work, a new approach in which the receiving structure is considered to have adaptive properties is studied. The aim is to control the transmitted vibrations by distributed arrays of piezoelectric transducers bonded to the receiving structure. The experimental rig consists of a rigid thick plate (the machine) supported at the corners by four elastic springs mounted on a thin clamped-free elastic steel plate (the receiving structure). The thick plate is driven by a harmonic force input. Response in the receiving panel is measured with a scanning laser vibrometer. Active inputs to the receiving structure are induced by three pairs of piezoceramic actuators bonded to the surface and configured to induce bending. The error sensors consist of up to two polyvinylidene fluoride (PVDF) strips attached to the panel surface in various positions. The control approach uses a two channel feedforward adaptive LMS algorithm implemented on a TMS320C25. The results show that the first three modes of the system can be controlled efficiently when driven ''on resonance,'' thus effectively isolating the vibrating structure from the ''machine'' raft input. However, when the system is driven ''off resonance,'' the vibrations of the receiving structure proved more difficult td reduce effectively. The paper presents vibration distribution of the receiving plate with and without control for a number of input frequencies as well as a variety of control transducer configurations.