PLIF: piezo light intelligent flea-new micro-robots controlled by self-learning techniques

1 . I N T R O D U C T I O N Microrobotics researches require to operate on autonomous devices with simple design and limited costs and energy consumption, even if this fact limits the possible tasks of each single robot. In the last few years the interest in micro mechatronic systems is increasing and several different kind of microrobots have been designed [1], [2],[3]. Applications of these robots can be found in micromachinig, inspection of small environments, micro-surgery, medicine [4], study of co-operating systems etc. In microrobotics particular attention should be paid in the selection of actuators, keeping in mind power consumption and size and weight of the components [5]. Among the different kind of actuators that have been adopted for microrobots we can mention pneumatic [6], electrostatic [7], shape-memory [8] and piezoelectric [9]. This paper wishes to increase the knowledge in this robotic area by choosing an innovating design for the locomotion of the robots. Our aim was to build walking microrobots, named PLIF (Piezo Light Intelligent Flea), that could be fast, agile small, light, cheap and autonomous in order to study experimentally self-learning strategies and collective behavior of robots. For this reason we have chosen piezoelectric actuators that are robust, strong, low-consuming and have a good displacement resolution [10],[11]. 2 . P I E Z O E L E C T R I C M A T E R I A L S Piezoelectric materials are characterized by their ability to produce charges when they are stressed and viceversa. This means they are even able to generate a force if a voltage difference exists between two faces of the piezoelectric bar [10],[11]. In particular we used piezoceramic bimorph elements that consists of two thin piezoelectric layers, 15 mm long, separated by a common electrode. When voltage difference is applied between the two terminals, one piezoelectric layer stretches while the other shortens so that the entire structure bends toward the shorter side, as is shown in Fig.1. In our operative conditions the maximum displacement that can be obtained is then ∆x=36μm. 3 . S T R U C T U R E D E S I G N 3.1 Mechanical part In our micro-robots the piezoceramic bimorphs have been adopted not only as actuators, but also as a part of each micro leg. In particular in order to build a walking micro-robot the simplest steady configuration has been chosen: two active legs moving and one passive as support. Each leg is composed by two parts realized by using two bimorph piezoceramic actuators each one 15mm long: the femur, that acts for the vertical movements of the leg, and the tibia that acts for the horizontal movements. Three different solutions of PLIF, schematically shown in Fig. 2, have been proposed. In the first prototype (PLIF I) the moving legs have been stuck externally to the body, the femur and the tibia of each leg make an angle of about 130 ∆ x Fig. 1. Piezoelectric effect on bimorph piezoceramic layers. degrees, which allows a more uniform grip on the ground. However the difficulties in assembling the pieces bring to an asymmetric structure that is unsuitable for a correct walk. PLIF I PLIF II PLIF III Fig. 2. The structure of the three different PLIF (front view, upper view and structure of a single leg). The second prototype (PLIF II, Fig. 3) is smaller (4cm of total surface occupied), having the legs fixed under the robot body; moreover the two pieces are orthogonally linked to prevent assembling problems.