Experimental testing of a modular flexible actuator based on sma wires

A flexible finger made up of three actuator modules based on shape memory wires (SMA) is experimentally studied in this research. A module is composed by few simple components: a plastic body and SMA wires. The body is a thin cylinder with a lower and upper base and two intermediate disks. Three equidistant SMA wires are longitudinally placed and allow the module to bend in any direction when one or more wires are actuated. The motion of the module is performed with the heating and cooling of the wire and the central rod exerts bias force, necessary to the stretching of the wire to the original length. Two test benches were built to perform both positioning tests and force tests. To evaluate the actuator workspace different tests were performed, with different power supply, heating and cooling time, actuation sequence. Force tests were performed with different distance between the undeformed finger and the obstacle. The results achieved with this first prototype are encouraging since the finger shows stable and correct operation. The planar projection of the workspace is a circle of about 30-40 mm of radius and exerted force is similar to mathematical model results (about 1 N at 5 mm). These results are encouraging, even though, probably due to manufacturing imperfections and frictions, the movement is not very regular along the various directions

Flexible Fingers Based on Shape Memory Alloy Actuated Modules

To meet the needs of present-day robotics, a family of gripping flexible fingers has been designed. Each of them consists of a number of independent and flexible modules that can be assembled in dierent configurations. Each module consists of a body with a flexible central rod and three longitudinally positioned shape memory alloy (SMA) wires. When heated by the Joule eect, one to two SMA wires shorten, allowing the module to bend. The return to undeformed conditions is achieved in calm air and is guaranteed by the elastic bias force exerted by the central rod. This article presents the basic concept of the module and a simple mathematical model for the design of the device. Experimental tests were carried out on three prototypes with bodies made of dierent materials. The results of these tests confirm the need to reduce the antagonistic action of the inactive SMA wires and led to the realization of a fourth prototype equipped with an additional SMA wire-driven locking/unlocking device for these wires. The preliminary results of this last prototype are encouraging

Fuzzy Logic Position Control of a Shape Memory Alloy Wire

Due to the thermomechanical characteristics of Shape Memory Alloy wires, it is important to develop control systems in order to design new applications for these smart materials. This work presents three SMA wire position controls: a classic PD control with PWM modulation is compared to two different fuzzy logic controls. They are implemented on a SMA wire (Flexinol®)with a diameter of 250 mm and a length of about 200 mm. The so called Fuzzy logic is particularly suitable in case of uncertain conditions and in presence of data acquisition noise and it is widely used to model and control time dependent and/or non linear processes. The experimental tests comprise square wave response tests, sinusoidal wave tests and multiple step response tests. Interesting results are a maximum error during stability phase with the fuzzy logic control of about 2%, four times smaller than that obtained with the PD control, with reduced fluctuations amplitude. The PD control with fuzzy supervisor is a control more simple than the fuzzy control and lead to similar results for the sinusoidal tests and multiple step response tests, with fluctuation amplitude of about 0,01 mm, much more less than those observed with the PD or the fuzzy control

Non-conventional lip seal mountings for pneumatic cylinders

Sliding lip seals are used in pneumatic cylinders to prevent leakage past the piston and rod. Though they guarantee excellent sealing, the friction forces at contact between lip and counterpart can be relatively high. This results in energy losses and problems with wear on components in relative motion, as well as difficulties in controlling actuator positioning. Standards for compressed air cleanliness restrict the use of greases and lubricants, especially in applications such as the food processing and pharmaceutical industries. A number of studies are thus under way in order to find effective alternatives to the use of common lubricants. These studies address both seal configuration [1-4], and the use of innovative materials [5-7]. Other approaches employ systems permitting small amounts of air leakage that operate as lubricated supports, variable-profile pistons [8], or rings with special micromachined surfaces [9-10]. Such solutions are not always economical, both because they require precision tolerances and geometries, and because they use special seals, often produced ad hoc. This paper discusses the possibility of using commercial lip seals with a non-conventional mounting on the piston in order to obtain a simple controlled-leakage system that reduces friction forces in pneumatic cylinders economically and effectively. The non-conventional mounting consists of positioning the lip seal in the direction opposite to that used in a conventional installation, so that the contact pressure reduces as air pressure in the chamber rises. In this way, an air gap is created at the sliding interface which allows air in the pressurized chamber to escape past the seal lip, thus reducing the contact force on the barrel. The piston is provided with a hole which exhausts the leakage flow to the atmosphere. To evaluate the validity of this approach, a number of preliminary tests were carried out in [11] to measure flow rate on a type of spring-loaded lip seal consisting of graphite-filled PTFE for barrel diameters of 50 mm. In particular, the tests determined air leakage behavior for different mounting tolerances. This paper presents further tests conducted on the same samples examined in [11] and on another PTFE and graphite-filled PTFE configuration, measuring leakage flow rates and friction forces. Leakage flow rate measurements were carried out with single and double seal installations. The results obtained are discussed together with the advantages of the proposed solution compared to the conventional mountings in ordinary use

Experimental analysis and preliminary model of non-conventional lip seals

The paper describes the tests carried out to determine the applicability of a simple means for reducing friction force in linear pneumatic actuators. The idea is based on the use of commercial lip seals mounted in the direction opposite to that used in conventional systems. Two different non-conventional installations were tested using commercial spring-energized seals consisting of two different materials: virgin PTFE and graphite-filled PTFE. Tests assessed the effect of bore manufacturing tolerances with nominal bore diameters of 50.0 mm and 50.2 mm. Performance was evaluated in terms of leakage flow rate and friction force, comparing the proposed installation with conventionally mounted seals. Friction coefficient and stiffness of the two seal materials were measured on a seal segment mounted on a pin-on-disk tribometer featuring a special pin design. Tests were carried out by loading the seal segment against a portion of the cylinder barrel in reciprocating motion. Stiffness and friction coefficient measurements served as the basis for developing a preliminary lumped parameter model of the lip seal which was used to analyze lip deflections, exchanged forces and leakage flow rate between seal and barrel

Analysis of the energy consumption of a rotary harrow

This paper shows the development and study of a FEM model of a rotary harrow. A proper use of a rotary harrow depends on its geometry and on the working parameters, such as: drag speed and angular speed. The aim of this work is to develop a rotary harrow model in the ANSYS environment in order to analyze and to optimize its geometric parameters. The model, validated through comparison with some experimental data, was then used to analyze the forces exchanged between the tines and the soil as a function of the drag speed, the angular speed and the working depth. Different tine orientations and three different types of terrain were considere

Study of the press forming mechanism of a thermoforming machine

The paper presents kinematic and dynamic investigations of the main press forming mechanism of a thermoforming machine. A multibody analysis of this press forming mechanism, which lifts and rotates a press bed, was carried out. Press bed lifting, which is necessary to form the component, is performed by means of a first rod and toggle mechanism. Press bed rotation to eject the formed component is produced by means of a second rod and must be appropriately shifted. These rods are oscillating followers driven by cams, making it possible to precisely define trajectories as a function of the motor shaft rotation angle. Analysis is performed by numerically solving the equations of motion. Cam synthesis on the basis of the oscillating followers' trajectories makes it possible to obtain cam profiles in order to evaluate pressure angles and check that there is no undercutting. System dynamics is investigated in order to evaluate motor torque and analyse internal stresses on the hinges. In addition, some experimental results and those obtained with the dynamic model are compared. Performance improvement of the actual machine is carried out by modifying the trajectory of the press bed by means of a numerical code at the purpose developed. This approach is more convenient than the use of a commercial multibody code, which is not specifically built for parametric studies