Alessandro Francesconi

Impact facility based upon high frequency two-stage light-gas gun

Abstract An impact facility based upon a two-stage high-frequency light-gas gun has been developed to allow fast and low-cost hypervelocity tests. The mechanical configuration and the managing electronic system are presented. The unit is powered only by means of high-pressure gas: no explosive powder is used. The system is managed by a dedicated computer system, which acquires signals from pressure transducers and operates nine electron valves. To improve the gun reliability, the control system has been designed to carry out an automatic diagnostic procedure after each shot. To improve the gun performance and the safety, an active piston-damping procedure has been developed. In this configuration a high shot frequency has been reached (10 shots/h ) . Projectiles with mass between 100– 300 mg have been launched up to 3– 4 km/s depending on the mass. This work can also be seen as a feasibility study for a new class of high-performance, high-frequency and low-cost two-stage light-gas guns, useful for the Italian Hypervelocity Laboratory, proposed to be built at the Italian Center for Aerospatial Research (CIRA).

Morphing electroadhesive interface to manipulate uncooperative objects

The possibility of handling uncooperative objects, i.e. objects not equipped with any features that can aid their manipulation, is of particular interest for both terrestrial and space robotic applications. In this framework, this paper deals with the development and testing of a smart material substrate, which can be integrated into an end-effector device, where morphing and electro-adhesive capabilities are combined to allow the manipulation of uncooperative objects of different shapes and materials. Compliance and adhesion properties are obtained by creating a conductive pattern of electrodes embodied on the surface of a polymeric substrate. On one hand, the polymeric material, activated by a change in temperature, can adapt to any shape when it is heated, and maintain the deformed shape after being cooled, even when the load is removed, becoming compliant with the objects surface. On the other hand, the conductive pattern is responsible for the adhesive effect: when a high voltage is applied, the electric field generated induces an opposite charge on the objects surface establishing reversible attraction forces. Furthermore, the conductive pattern could be used to activate the morphing behaviour when the manipulator and the target object come into contact. A resistiveelectroadhesive pad is realized and some tests are performed to verify the heating behavior of the electrodes and the electroadhesion forces achievable. Morphing tests are also performed to verify the ability of the polymeric substrate to maintain the deformed shape after cooling.

2D Close-Range Navigation Sensor for Miniature Cooperative Spacecraft

This paper presents the 2D close-range navigation sensor for cooperative spacecraft developed in the framework of the experiment ARCADE. The sensor exploits an infrared LED transmitter mounted on the target, and two photodiode receivers mounted on the chaser. Laboratory tests were performed to assess the sensor performance and range of application. The accuracies of estimates were, on average, 1.9 mm for distance and 1.0 deg for yaw angle, in ranges of [0.20, 0.42] m and [-40, 40] deg, respectively.

A system to damp the free piston oscillations in a two-stage light-gas gun used for hypervelocity impact experiments

Hypervelocity impact experiments that reproduce on-orbit collisions between micrometeoroids or orbital debris and space structures are commonly performed by means of propellant-driven two-stage light-gas guns. Such devices accelerate projectiles using the thrust of a light propellant gas that is compressed to high pressure and temperature by a piston running in a pump tube. Though these guns have the unique capability of accelerating particles up to 9 km/s, many components of the gun must be checked and/or substituted after each shot making test sessions long and expensive. In order to have a lot of and many different types of hypervelocity impact data, the Center of Studies and activities for Space CISAS “G. Colombo” of Padua University developed a high-shot-frequency two-stage light-gas gun that can increase the shot repetition rate of standard facilities by a factor of 5 or more and at the same time reduce the shot cost by a factor of 2 or more. This is made possible through the use of special mechanic...

Modelling and control of double-cone dielectric elastomer actuator

Among various dielectric elastomer devices, cone actuators are of large interest for their multi-degree-of-freedom design. These objects combine the common advantages of dielectric elastomers (i.e. solid-state actuation, self-sensing capability, high conversion efficiency, light weight and low cost) with the possibility to actuate more than one degree of freedom in a single device. The potential applications of this feature in robotics are huge, making cone actuators very attractive. This work focuses on rotational degrees of freedom to complete existing literature and improve the understanding of such aspect. Simple tools are presented for the performance prediction of the device: finite element method simulations and interpolating relations have been used to assess the actuator steady-state behaviour in terms of torque and rotation as a function of geometric parameters. Results are interpolated by fit relations accounting for all the relevant parameters. The obtained data are validated through comparison with experimental results: steady-state torque and rotation are determined at a given high voltage actuation. In addition, the transient response to step input has been measured and, as a result, the voltage-to-torque and the voltage-to-rotation transfer functions are obtained. Experimental data are collected and used to validate the prediction capability of the transfer function in terms of time response to step input and frequency response. The developed static and dynamic models have been employed to implement a feedback compensator that controls the device motion; the simulated behaviour is compared to experimental data, resulting in a maximum prediction error of 7.5%.

Kinematics and control of redundant robotic arm based on dielectric elastomer actuators

Soft robotics is a promising field and its application to space mechanisms could represent a breakthrough in space technologies by enabling new operative scenarios (e.g. soft manipulators, capture systems). Dielectric Elastomers Actuators have been under deep study for a number of years and have shown several advantages that could be of key importance for space applications. Among such advantages the most notable are high conversion efficiency, distributed actuation, self-sensing capability, multi-degree-of-freedom design, light weight and low cost. The big potentialities of double cone actuators have been proven in terms of good performances (i.e. stroke and force/torque), ease of manufacturing and durability. In this work the kinematic, dynamic and control design of a two-joint redundant robotic arm is presented. Two double cone actuators are assembled in series to form a two-link design. Each joint has two degrees of freedom (one rotational and one translational) for a total of four. The arm is designed to move in a 2-D environment (i.e. the horizontal plane) with 4 DoF, consequently having two degrees of redundancy. The redundancy is exploited in order to minimize the joint loads. The kinematic design with redundant Jacobian inversion is presented. The selected control algorithm is described along with the results of a number of dynamic simulations that have been executed for performance verification. Finally, an experimental setup is presented based on a flexible structure that counteracts gravity during testing in order to better emulate future zero-gravity applications.

Design of an innovative Dielectric Elastomer actuator for space applications

The capability of Dielectric Elastomers to show large deformations under high voltage loads has been deeply investigated to develop a number of actuators concepts. From a space systems point of view, the advantages introduced by this class of smart materials are considerable and include high conversion efficiency, distributed actuation, self-sensing capability, light weight and low cost. This paper focuses on the design of a solid-state actuator capable of high positioning resolution. The use of Electroactive Polymers makes this device interesting for space mechanisms applications, such as antenna and sensor pointing, solar array orientation, attitude control, adaptive structures and robotic manipulators. In particular, such actuation suffers neither wear, nor fatigue issues and shows highly damped vibrations, thus requiring no maintenance and transferring low disturbance to the surrounding structures. The main weakness of this actuator is the relatively low force/torque values available. The proposed geometry allows two rotational degrees of freedom, and simulations are performed to measure the expected instant angular deflection at zero load and the stall torque of the actuator under a given high voltage load. Several geometric parameters are varied and their influence on the device behaviour is studied. Simplified relations are extrapolated from the numerical results and represent useful predicting tools for design purposes. Beside the expected static performances, the dynamic behaviour of the device is also assessed and the input/output transfer function is estimated. Finally, a prototype design for laboratory tests is presented; the experimental activity aims to validate the preliminary results obtained by numerical analysis.

Design, qualification, and acceptance of the front-door mechanisms for the OSIRIS experiment of the Rosetta mission

The scope of the present document is to describe the Front Door Mechanism of Wide Angle Camera and Narrow Angle Camera of the OSIRS payload and to demonstrate the safety of the mechanism itself. To match the scientific goals it has to prevent the contamination of the telescopes during ground and in -flight operations. Moreover for the in-flight calibration the door itself will be used for the flat field of the cameras. The original design has been carried out according to optimisation techniques and successfully tested during the qualification and acceptance campaign. The results obtained demonstrated that the mechanism is fully compliant to the interface specifications and its performances are not affected by the environmental stresses and by the overall life cycling.