A. Manuello Bertetto

Characterization and modeling of air muscles

Abstract Two flexible pneumatic actuators, McKibben and straight fiber, were analyzed. The muscles were modeled by finite element method. The numerical models take into account the non-linearity of the expanding rubber inner tube, and of the mechanism for transferring load to the braided cords surrounding the tube. Because of its own fabrication, McKibben muscle model may capture the initial backslash between rubber tube and cords. Despite of the McKibben specimen, a straight fiber muscle prototype was fabricated on purpose in the laboratory. In order to validate the numerical models, numerous experimental tests were carried out. The numerical results showed a good agreement with the data measured experimentally in terms of muscle pull force and of its deformed shape. Finally, the models of two muscles similar in terms of input energy allow their performances to be compared.

Experimental and numerical study of friction in an elastomeric seal for pneumatic cylinders

Abstract The behaviour of an elastomeric seal for a pneumatic cylinder piston is analysed in this work. The aim of the paper is to validate experimentally a numerical approach to evaluate seal performance, in actual working conditions. The friction force between seal and the cylinder bore is evaluated through experimental tests and numerical simulations, using a finite element model. Working conditions are simulated for different piston rod velocity and cylinder supply pressure, both in dry conditions, boundary and fluid lubrication conditions. Simulation is obtained by an iterative process that uses the friction coefficient, experimentally measured, as a function of speed and lubrication conditions. Results obtained from experimental tests and numerical simulation are compared to determine the friction force between seal and cylinder bore.

Photoelastic measurements and computation of the stress field and contact pressure in a pneumatic lip seal

Abstract This work regards the analysis of the structural behaviour of a typical elastomeric lip seal for pneumatic cylinders under actual working conditions. In situations of relative motion between the lip seal and the cylinder, coulombian friction is taken into account. The analysis has been carried out by means of both numerical and experimental methods. In particular, the behaviour of the seal with respect to the two contact zones, i.e. between seal and rod and between seal and mounting housing, has been studied. In order to describe the seal stress and strain fields, isoclinic lines and isochromatic fringes have been determined both by means of a two-dimensional finite element model and by reflection and transparent photoelastic methods. The contact pressure between the lip seal and the rod has been computed numerically, with the FEM model and analytically from photoelastic data. The comparisons between experimental and numerical data have permitted numerical model validation.

A Two Degree of Freedom Gripper Actuated by SMA with Flexure Hinges

A gripper prototype was designed and built. It is made by a rigid structure articulated by compliant hinges. Its kinematics consists of both parallel and angular finger motion. The movements were designed to be independent from each other and auto-adaptive as well. The motions were driven by Ni-Ti shape memory alloy (SMA) wires. The recovery position is achieved by the elastic force exerted by the flexure hinges in the case of parallel motion and by an axial spring in the case of angular motion. Both the actuators and the hinges were experimentally characterized by suitable test rigs. The gripper prototype was tested and it showed to be able to reach the design performances.

One-dimensional experimental mechanical characterisation of porcine aortic root wall.

The human aortic valve, in cases of disease, can be substituted with a stentless biological prosthesis that is made of both porcine aortic root and leaflets. In particular, the aortic root plays a very important part in the opening mechanics of the valve. Therefore, to understand the behaviour of the prosthesis, a knowledge of the mechanical characteristics of each element constituting the system is useful. For a structural and mechanical characterisation of the porcine aortic root wall tissue, specific measuring devices are made. A uni-axial tensile apparatus, operating in a temperature-controlled saline bath, is equipped with special pneumatic clamps. A test rig for the measurement of the specimen cross-sectional area is developed. To determine the porcine aortic root wall mechanical properties, 189 tensile tests are carried out. Specimens of both natural and chemically fixed tissue are used to consider the conditions of both natural valves and prostheses. Tensile tests are carried out on both axial and circumferential specimens taken from 12 areas of one-sixth of the entire aortic root, with the aim of identifying the anisotropic and non-homogeneous behaviour of the tissue. The behaviour changes considerably depending on the direction in which the specimen is strained, the chemical treatment and the zone from which it has been taken. The paper presents the stress-strain characteristics of fresh and fixed tissue in all zones of the aortic wall, both in circumferential and axial directions.

An integrated device for saffron flowers detaching and harvesting

This work is concerned with a mechanical system designed to harvest Crocus Sativus (saffron) flowers. The system is conceived as a shoulder portable device with two main parts: the first one is specifically designed to detach the flower containing three stigmas, which are the costly final product; the second one is aimed to collect the detached flower through a vacuum collector. The system is carried by the human operator on his back. This paper describes the operating principle of the detaching and of the harvesting devices. The former device imitates one of the procedures followed by the pickers but with the peculiarity that allows to harvest the flower without separating it from its leaves, which is a significant advantage since it simplifies the mechanical detachment of the flower. The paper also deals with some experimental tests in the laboratory carried out in order to highlight the dynamic behaviour of the detaching and of the harvesting devices.

Contact pressure distribution in guide bearings for pneumatic actuators

This paper deals with the analysis of the contact pressure distribution at the rod/guide bearing interface of a linear pneumatic actuator. The investigation was carried out both experimentally, using pressure-sensitive film, and numerically by means of finite element analysis. By using the numerical model, it was possible to identify design changes to the cylinder front head whereby contact pressure at the bearing/rod interface can be redistributed. Operating conditions that are more advantageous in terms of wear and durability can thus be achieved.

Low cost resistive based touch sensor

Abstract In this paper a resistive based sensor is presented. The transducer is made of a rubber sample as compliant material with a graphite film spread on the whole surface as conductive material. An extensive experimental program was carried out in order to characterize the device. Both quasi-static and fully dynamic responses were measured by means of dedicated test rigs. Moreover, a neural network model of the sensor was developed in order to capture its transfer function. A good agreement was found between the sensor responses obtained by the experiments and these according to the numerical model.

A model of contact forces in pneumatic motor vanes

The paper presents an investigation on the dynamic equilibrium of a pneumatic motor vane. The investigation was based on an analysis of the vanes free body diagram. In particular, a model was formulated for determining the contact forces between vane, rotor and stator and the contact configuration from the known motor geometry, friction coefficients and chamber pressures. In addition, the paper presents experimental measurements of friction coefficients in vane-rotor and vane-stator contact together with a validation of the model through comparison with a FEM model.

Experimental study of pneumatic seal extrusion

An experimental study of pneumatic seal extrusion is presented. Direct visual observations are made. An empirical formula for calculating extrusion based on the geometric characteristics of the sealing system is presented.