Resonant Excitation of a Composite Beam Using Piezoelectric MFC Actuators

Due to leakage resistances measuring slowly changing loads using piezoelectric sensors is rather difficult while measuring static loads is impossible. Using the functional principle of a scale with a vibrating string a work-around to this problem is found. By evaluating the relation between the load onto the string and the eigenfrequencies of bending vibration of the string the external load onto the string can be determined. Instead of a string of circular cross-sectional area a thin beam can be used which leads to a preferential vibration direction in direction of least bending resistance. To achieve large signal amplitudes the beam should be excited and vibrating close to its eigenfrequency. To remain in a state of resonance at all times the frequency of excitation must follow frequency alterations due to external influences. Such a frequency locking can be achieved by applying phase-locked loops (PILL) which are widely used within communication technology. In this contribution an experimental set-up for resonant excitation of a composite beam is presented. A small mass is mounted onto the beam. Its position is adjustable to modify the eigenfrequency of the beam. Using piezoelectric macro-fiber composites for sensing and actuating as well as an integrated PLL circuit the resonant excitation of the beam can easily be achieved even for changing eigenfrequencies.