| dc.description.abstract | Unmanned Aerial Vehicles (UAV) are essentially automatic flight vehicles having dimensions, wingspan and airspeed, smaller than the conventional aerial vehicles. UAVs are employed widely in applications such as surveillance over a short distance, acquisition of a local target, detection of hazardous chemicals / biological agent, exploration of a harmful environment, search operations, etc. UAV can be classified into three main types depending on their method of propulsion and lift. These are fixed wing, rotary wing, and flapping wing. Flapping wing UAVs are more suitable for insect scale flights.
Flapping mechanism requires actuators with large stroke periodic (reciprocal) motion at high speed (10-100s of Hz) with large output forces for overcoming the aerodynamic damping. There are several actuation mechanisms applicable to flapping-wing UAVs. The emphasis is on linear actuators, which simplify the mechanical transmission for flapping motions. Most of the prototypes developed so far have employed motor-driven mechanisms to achieve the flapping wing. Unconventional methods such as piezoelectric, thermal, electromagnetic, shape memory, electrostatic, etc. for actuation of flapping wings have also been used. Among the unconventional actuation methods, piezoelectric actuation is the most used mechanism because of its compact size and high-power density.
However, the deflection generated by the piezoelectric actuator is intrinsically very small. Therefore, it is necessary to employ numerous types of motion amplification mechanisms to achieve large deflection. The development of an amplification mechanism is a complex procedure. Hence, several efforts have been made to evaluate different forms of piezoelectric actuators for flapping vehicle application.
The in-situ piezoelectric actuator, employing piezoceramic coating directly on the structure by a simple method, looks promising for flapping wing application as it generates large displacement compared to the displacement produced by the bulk piezoelectric actuators. Piezoceramic coating is light in weight compared to the bulk piezoelectric actuator, as it is a composite material of highly dense piezoceramic material and less dense polymeric material. The current study evaluates the basic characteristics, performance and the aerodynamic behavior of the in-situ piezoceramic actuator for flapping wing applications.
Initially, the basic characteristics of the in-situ piezoceramic actuator were evaluated. Its properties like density, elastic modulus and the transverse piezoelectric coefficient, d31, were evaluated. The density was measured to be 2300 kg/m3. Elastic modulus was measured as 3 GPa. The piezoelectric coefficient, d31 was measured by applying the coating on various substrates such as stainless steel, brass and polymer. Also, the in-situ piezoceramic actuator was applied with different thicknesses 30, 40 and 50 µm on the polymer substrate. The mean value of the measured d31 was found to be -26 pm/V.
The performance of the piezoceramic coating (coated on a flexible substrate) was evaluated for flapping wing application using performance metrics from aerodynamic studies. The performance metrics are the tip velocity,, the dynamic electromechanical coupling factor (EMCF), k12, and the elastic energy per unit mass, . These metrics were obtained and verified using an analytical model. Based on these metrics, the performance of the developed flexible actuator was compared with conventional flexible piezoelectric polyvinylidene difluoride (PVDF) actuator. These metrics were found to be better for the developed actuators than PVDF.
The best suitable wing configuration was selected using FE modelling based on the tip velocity ( ). It was found that the tip velocity depends on the thickness, length and shape of the flapping wing. Wing shapes of the dragonfly’s, tobacco hawkmoth’s and cicada’s forewings were considered. The thickness and length of the in-situ piezoceramic actuator were also varied. The results obtained through FE modelling were verified experimentally. Dragonfly wing was found to give maximum tip velocity ( ). The maximum value for υ of 114.7 mm/s was obtained for a dragonfly wing having in-situ piezoceramic actuator of 30 mm length from the root of the wing and a thickness of 30 µm. The lift force was measured using a load cell measurement set up in the clamped condition for the dragonfly wing.
Insects use a variety of wingbeat kinematics to produce and control aerodynamic forces for their flight. For mimicking an insect flight, the selected actuator needs to be capable of producing insect flight kinematics. Therefore, twisting along with flapping motions of the wings by the in-situ piezoceramic actuators were attempted. Twisting motion of the wing was achieved by actuating two piezoceramic cantilevers type in-situ actuators applied over a wing with sinusoidal signals out of phase with each other by 180°. The fore wing and the hind wing were actuated by sinusoidal signals with a phase difference of 0°, 90°, 180° and 270°. Tip displacements of 4 mm for the fore wing and 3 mm for hind wing were measured. The kinematics of the flapping wing, which has been achieved by other actuators with complex mechanisms can thus be achieved by the simple in-situ piezoceramic actuators. The experiments on the measurement of the lift and kinematics of the flapping wing establishes that the in-situ piezoceramic actuator is a suitable candidate for flapping wing application.
The improvement studies on tip velocity were carried out by implementing in-situ bimorph piezoceramic actuators. Dragonfly wings with the piezoceramic layer of thickness 30 µm were considered. As there was a implication of increase in the mass of the wing, selection of length of piezoceramic layers were carried out using FE modelling. The results obtained through FE modelling were verified experimentally. The maximum tip velocity of 245.1 mm/s is obtained for 25 mm length of both piezoceramic coating layers.
Modified strip theory based on the blade elemental analysis has been used to study the aerodynamic performance of the three types of wing planforms. Lift, thrust and drag forces generated by the three wing forms have been calculated analytically by the model. As flapping wings are operated at the first mode resonance, the first mode resonant frequency, and the tip displacement at resonance from the experimental results were given as input to the aerodynamic model. The effect of variations in aerodynamic parameters such as incident angle, pitching angle and forward speed on the relevant forces were studied for the in-situ piezoceramic actuator actuated wings of three different wing planforms. For all the three wing planforms, it was observed that the lift increased with incident angle and forward speed and it was less affected by increasing the pitching angle. For the increasing forward speed, incident angle and pitching angle, thrust and drag also increased. The modelling results show that the wings produce positive mean lift and condition where the thrust is more than the drag for all the wing planforms.
The overall results show that the in-situ piezoceramic actuator can be employed for flapping wing applications with further efforts. | en_US |
