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TRANSFORMATION OF ENERGY IN PIEZOELECTRIC DRIVE SYSTEMS.
Abstract
Macroscopic piezoelectric drive systems allow the transformation of energies up to 37 mW/cm**3. The fundamental principles of drives operating in the extensional mode and in the flexural mode are calculated and a bimetal flexural drive is optimized.
It is shown that the effectiveness of a heterogeneous bimorph in converting electrical energy into mechanical work is a function of the piezoelectric coupling factor, the thickness ratio and the elastic compliance ratio between the nonpiezoelectric and piezoelectric material comprising the bimorph. It is found that the heterogeneous bimorph has the largest effectiveness in performing work against a constant load applied as a moment. Effectiveness of the bimorph in performing work against a constant load applied as a force at the free end is larger than that of an applied pressure
A piezoelectric heterogeneous bimorph is configured as a cantilever beam, clamped at one end, and subjected to an applied voltage V and three mechanical boundary conditions at the free end: a mechanical moment M, a perpendicular force F, and a uniform body force p.A work cycle to transfer energy is assumed for the bimorph. The electrically and mechanically stored energies and the work performed by the bimorph against the load are calculated at different states of the work cycle. The stored energies and the performed mechanical work are related to the electrical energy taken in by the bimorph. The effectiveness λ for a constant load is defined as the ratio of the energy transferred by the bimorph to the energy put in by the generator. Optimal thickness ratio and elastic compliance ratio between the non-piezoelectric and piezoelectric materials for maximum effectiveness are presented.
A piezoelectric motor unit has been designed, which consists of a pair of cantilever beams of silicon, silicon nitride, polysilicon or other suitable material, on the top surfaces of which piezoelectric films have been deposited, whose free tips are connected by a structure that allows this pair of beams to ‘walk’, or if the structure is restrained, to exert a force. This force can be used to drive a circular gear or linear slider structure. This motor unit is expected to be able to provide an alternative to the electrostatic motor for the generation of motion in a micromechanical assembly. A piezoelectric film covering a cantilever beam is able to move the tip of the beam in a direction perpendicular to the plane of the beam. A set of two juxtaposed beams forms a motor unit; the individual beams can move in opposite directions, while a suitably designed rotatable structure between their tips is able to provide a motion in the plane of the beams and perpendicular to their length. These pairs of cantilever beams could be used to form ‘legs’ with which ‘gnat robots’ or microrobots could walk; alternatively they could be arranged to form ‘jaws’ or other tools with which the robot could grab objects and manipulate them. As these robots are made of silicon, they could have on-board processors to coordinate the movements of their limbs, and they could be powered by on-board solar cells, acoustic receivers, etc. and could obtain instructions by means of acoustic signals or otherwise. Designs of micromotors and instruments are presented, designs of microrobots that can make insect-like or caterpillar-like movements will be shown, and a numerical analysis of their expected performance will be given.
A brief review of applications of piezoelectric bimorphs is presented. The constituent equations which describe the behavior of piezoelectric bimorphs for various mechanical boundary conditions are derived. The internal energy density of infinitesimally small volume elements in thermodynamic equilibrium is calculated in the presence of a voltage on the electrodes, a clamped cantilever beam condition on one side of the beam and a set of three different classical boundary conditions on the other side of the beam. These are a mechanical moment M at the end of the beam, a force F perpendicular to the beam, applied at its tip, and a uniformly distributed body force p.The total internal energy content is calculated by integrating over the entire volume of the beam. Two different beam configurations are considered: parallel polarizations of the two adjoining elements of the beam with an internal electrode; and antiparallel orientation without an internal electrode.The canonical conjugate of the moment is calculated as the angular deflection at the tip of the beam α, while that of the force at the tip is the local vertical deflection δ. The canonical conjugate of the uniform load on the beam is found to be the volume displacement V of the beam. The canonical conjugate of the voltage across the electrodes is the charge on the electrodes. The equations are given in the direct form, with external parameters (M, V), (F, V), and (p, V) as independent variables and also in a linear combination with (M, F, p, V) as variables.These constituent equations can be used to calculate the behavior of the bimorph under any condition that can be described as a linear combination of forces at the tip, moments at the tip and uniform loads on the entire beam. This allows us to use the bimorph as a black box, without having to consider its internal movement or charges.
This paper presents the design, fabrication and characterisation
of a piezoelectric microactuator array, the key component of a remotely
controlled, wireless, solar powered microsystem which should be capable
of locomotion. The walking principle mimics the elliptical leg motion of
walking animals by using a combination of multiple DOF surface
micromachined piezoelectric actuators associated with wafer-through
etched features that transmit and amplify the piezo-induced motion to
the backside of the wafer. Aluminium nitride has been chosen as the thin
film piezoelectric material since it is highly insulating and can be
deposited repeatedly with very high quality. The characterisation of the
fabricated structure shows very promising results in view of meeting the
wireless locomotion challenge
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