A tether-less legged piezoelectric miniature robot using bounding gait locomotion for bidirectional motion Hassan Hussein Hariri, Leonardus Adi Prasetya, Gim Song Soh, Shaohui Foong, Kevin Otto and Kristin Wood Abstract— This paper describes the design and evaluation of a Legged Piezoelectric Miniature Robot (LPMR) propelled by standing wave locomotion where the vibrations of legs are similar to the bounding gait locomotion of animals. The LPMR comprises of one piezoelectric patch, a metal beam, two contact joints, two rigid legs and contains all necessary power electronics required for tether-less operation. Through analysis of the bending modes of vibrations and driving frequency, a forward and backward motion is achieved by choosing specific positions for the legs. At 100 V amplitude and without embedded mass, the LPMR with the weight of 6.27 g, the length of 50 mm, the width of 10 mm and the height of 1.5 mm achieves maximum linear speed of 246.5 mm/s for forward motion and 302 mm/s for backward motion. The LPMR is able to carry 100.8 g at a speed of 49.6 mm/s for forward motion and 87.9 mm/s for backward motion when applying 100 V amplitude. The LPMR has a blocking force of 12 mN for forward motion and 9.8 mN for backward motion at 100 V amplitude. An experimental characterization for the LPMR in terms of speed versus applied voltage, speed versus embedded mass and blocking force for different applied voltages is explored and evaluated in this study. I. INTRODUCTION Piezoelectric materials are widely used in miniature mobile robots for actuation and sensing due to their abilities to generate forces when electric voltages are applied to them and vice versa to produce voltages when forces are applied to them [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. They are characterized by high power to weight ratio which make them particularly suitable for meso or micro scale mobile robots [12]. The drawback of piezoelectric materials due to requirement of onboard high driven voltage is no longer a problem nowadays as many works are reported in literature to overcome this issue [13], [14], [15], [16], [17], [18] which now makes it possible to integrate onboard electronics at small scale for driving piezoelectric miniature robots. In this paper, we focus on Legged Piezoelectric Miniature Robots (LPMR’s). Two types of LPMR’s exist in contemporary literature. The first type of LPMR’s uses active legs where the legs are designed from piezoelectric materials [3], [19], [20], *The authors gratefully acknowledge the support of the SUTD Temasek Laboratories sponsored project Systems Technology for Autonomous Re- connaissance & Surveillance (STARS) and the SUTD-MIT International Design Center (http://idc.sutd.edu.sg). Authors are with the Singapore University of Technology and Design, Engineering Product Development Pillar, 8 Somapah Road, Singapore 487372. [21], [22]. The second type of LPMR’s uses passive legs where piezoelectric actuators are used to create the motion of the passive legs [6], [16], [17], [18], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]. The LPMR described in this paper is subscribed in the second type where passive legs are used. It consists of one piezoelectric patch, a metal beam, two contact joints and two rigid legs as shown in Fig.1. The piezoelectric patch is bonded on the metal beam Fig. 1. Side view of LPMR structure to form a unimorph piezoelectric actuator. The two rigid legs are attached to the metal beam through the contact joints. The purpose behind the use of a single piezoelectric patch in the design of the LPMR is to reduce the complexity of the onboard electronics, control and power consumption. The LPMR in this research is easy to manufacture as it consists of a unimorph piezoelectric actuator that acts both as the base body and the excitation source for locomotion and two legs attached to it by rigid contact joints. Also, it doesn’t require any linking mechanisms. We chose an asymmetrical design placement for legs to achieve a bidirectional motion. The legs of our LPMR are rigid and the motion direction is determined by the legs positions and the corresponding vibration modes of the beam. The design exploits the use of mechanical standing waves generated on the metal beam and transmitted to its legs to propel the LPMR forward and backward where the vibrations of legs are similar to the bounding gait locomotion of animals. In our proposed design, a forward and backward motion in one dimensional axis is determined by the mechanical design and placement of the legs of the LPMR, and two operating frequencies for both directions are selected by the designer as well. By simple change in the driving frequency, both forward and backward motion can be achieved on a smooth flat surfaces.