The helicopter cannot fly at the height from 0 to 300 m by the aviation law, natural disasters and vegetation information on the ground need to be observed and photographed from the air conveniently, controllable flight devices close to the ground are demanded urgently. Wireless controllable helium balloon system has characteristics of carrying heavy things, long battery life, low cost and noise, fast, energy saving and safe. Although some progress has been made on the bionics application, the understanding of the flight mechanism of robots based on dielectric elastomer actuators (DEA) is not sufficient. Therefore, design the reasonable structures and movement rules, carry out theoretical computation and experimental research, analyze the force, power and other key issues for the robot motion, all which will help to develop the flight mechanism and provide references for the soft-drive vehicles in the future.
The flight robot base on DEA could realize the up and down, rotational and translational motion, calculation models were established, the influence factors on the buoyancy, the resistance and the trajectory were studied through measurements, experiments, theoretical derivation and simulation. The main contents and conclusions are as follows:
(1) Study on the reasonable structure and volume change of balloon actuators with electromechanical properties and carrying capacity. An information collection platform was built, meanwhile, high-speed cameras were used, the critical data of parameters was obtained after the software analysis and calculation. A flying robot system based on DEA was established using obtained data; Formulas were derived to describe real-time variations of pretension, voltage, pressure, charge and volume. The variation of the Helmholtz free energy equals the work done by voltage, pressure and the inertia, combining with the Maxwell stress, models with key parameters are obtained. Under the effect of electric field exists the electromechanical instability and electric breakdown phenomena on the DE film, electric breakdown relates to the prestretch closely, with increasing voltage, the electromechanical instability happened, wrinkles formed either near the clamped edge or the top part in the inflated balloon whose pre-stretch coefficient λpre was 3 to 6, the voltage Φ was greater than 5.5 kV.
(2) Unstable phenomena on the DE surface, such as loss of tension and electric breakdown. In order to imitate biological adhesion performance and skin properties, dielectric elastomers applied high voltages are used. A DE membrane could undergo wrinkles or bulging from the flat state when a critical voltage is applied. Parameters of the theory were obtained by fitting the experimental records. Four experimental phenomena were found out before electric breakdown in membranes supported by a circular frame: The active region in a membrane expanded against the passive region, the membrane was still flat till breakdown; Bulging formed on the membrane; Wrinkles and bulging coexisted; Wrinkles formed without bulging; Additionally, in the fourth state, two types of transition between flat regions and wrinkled regions were observed: Wrinkles formed in small regions, then they grew at the expense of flat regions until the entire active region became wrinkled, the process was continuous; Both wrinkled and flat regions moved interchangeably on the membrane till breakdown. It is found that the prestretch affects electromechanical phase transitions significantly.
(3) The structural design of the DE balloon actuators with large deformation . When the DE spherical shell is subject to an electric field through its thickness, it expands in radial direction and contracts in thickness. On the experimental platform, the pressure and displacement sensors were used to measure DE balloons. The influence of parameters, such as expansion amplitude and vibration frequency of soft spherical shells, was presented and analyzed on the buoyancy, the resistance and movements. Based on theoretical analysis, some spherical structures filling with helium or air are designed. The main conclusions of these studies include: Under the same pressure, the volume of the DE helium balloon is smaller than air balloon; The large deformation is very important for controlling DEA robots, we can increase the initial volume, electrode density and preloading to achieve this aim. The greater volume changes, the more weight can be taken by a helium balloon, and the more buoyancy force could be controlled. By analyzing the dynamic behavior of spherical shells, the thick ones deformed smaller than thin shells. Finally, A large number of experiments demonstrated the feasibility of the controllable flight for the DEA robot.
(4) Based on DE balloon actuators, the buoyancy，the resistance and power consumption of the robot were studied. Computational models of the DEA robot were established. The quality, speed, acceleration and other parameters of the system were calculated out. The relation between the critical breakdown voltage and the prestretching radius were obtained which explored the Gent model to derive the equation of volume and pressure combining the ideal gas law. The simulation was given to describe the change trend while the natural rubber coupled with VHB. Compared with experiments, the influence of the parameters, such as DC voltage, AC amplitudes, frequency, was analyzed on the moving direction of the robot. The breakdown voltage range of natural rubbers is generally between 1.5 kV and 5.5 kV, mostly at about 2.5 kV, which relates to the thickness of rubber. When inner rubber is blast, the volume change of the DE balloon is relatively giant, therefore, the buoyancy and acceleration of the DEA system are produced, the soft-driven robot will stay at a certain level in the sky.
(5) The controllable up-and-down, rotating and translational movement of the DE actuated robot. Considering the actual structure effect on robot motion, models of voltage, pretension and volume were established. The moving trajectory was analyzed, and experimental results verified the accuracy of the models. Combined with inertial power consumption, moving directions were analyzed under the effect of DC and AC voltage, the robot can realize wireless control in rotational and translational motions, etc. The DE balloon actuator was connected to a large and non-elastic chamber with carrying wireless controlling system and loads. The flight acceleration was obtained by calculating the system volume change. In order to realize the controlling strategy and coordinate each controller, we put forward the main controller, soft actuated controller and the tail part as independent agents, the mechanism of communication and collaboration between each part were given; The information exchange format was developed, and the software design process was explained. In addition, the controllable buoyancy of the robot decreases with the increase of flight height, the temperature and the humidity in the atmosphere.