An Innovative Mechanical and Control Architecture for a Biomimetic Hexapod for Planetary Exploration M

An Innovative Mechanical and Control Architecture for a Biomimetic Hexapod for Planetary Exploration M. Pavone∗, P. Arenax and L. Patane´x ∗Scuola Superiore di Catania, Via S. Paolo 73, 95123 Catania, Italy xDipartimento di Ingegneria Elettrica Elettronica e dei Sistemi, Universita` degli Studi di Catania, Viale A. Doria, 6 - 95125 Catania, Italy Abstract— This paper addresses the design of a six locomotion are: wheels, caterpillar treads and legs. legged robot for planetary exploration. The robot is Wheeled and tracked robots are much easier to design specifically designed for uneven terrains and is bio- and to implement if compared with legged robots and logically inspired on different levels: mechanically as led to successful missions like Mars Pathfinder or well as in control. A novel structure is developed Spirit and Opportunity; nevertheless, they carry a set basing on a (careful) emulation of the cockroach, whose of disadvantages that hamper their use in more com- extraordinary agility and speed are principally due to its self-stabilizing posture and specializing legged plex explorative tasks. Firstly, wheeled and tracked function. Structure design enhances these properties, vehicles, even if designed specifically for harsh ter- in particular with an innovative piston-like scheme rains, cannot maneuver over an obstacle significantly for rear legs, while avoiding an excessive and useless shorter than the vehicle itself; a legged vehicle, on the complexity. Locomotion control is designed following an other hand, could be expected to climb an obstacle analog electronics approach, that in space applications up to twice its own height, much like a cockroach could hold many benefits. In particular, the locomotion can. Secondly, wheeled and tracked vehicles are also control is based on a Cellular Neural Network playing inherently less robust than those dependent on legs. the role of an artificial Central Pattern Generator. Several dynamical simulations were carried out to test The loss of a single leg on a hexapod will result in the structure and the locomotion control. Simulation only minimal loss in maneuverability, i.e. in a me- results led to the implementation of the first prototype: chanical graceful degradation; on a wheeled vehicle Gregor I. Experimental tests showed that Gregor I is a damaged wheel could spell the end of mobility, and able to walk at the travel speed of 0:1 body length a damaged caterpillar tread almost always results in per second and to successfully negotiate obstacles more catastrophic failure. Finally, legged vehicles are far than 170% of the height of its center of mass. more capable of navigating an intermittent substrate -such as a slatted surface- than wheeled or tracked I. INTRODUCTION vehicles [1]. That is why the concept of a fully autonomous, The recent planetary explorative missions as well mission capable, legged robot is acquiring an ever as the scheduled ones for the next years put in light increasing interest in the field of space explorative the necessity, for planetary unmanned missions, of robotics. using autonomous mobile platforms. Given the preceding arguments for the use of Mechanical structure is the first issue to be ad- legged locomotion in certain environments, one is dressed. Possible mechanisms capable of producing left with the challenging task of actually designing * Corresponding author. E-Mail: [email protected] an efficient locomotion control for legged robots. Marco Pavone is currently a Ph.D. student at the University of The basic consideration is that legged structures California, Los Angeles, Mechanical & Aerospace Engineering, have a great number of degrees of freedom, to be Los Angeles, CA 90095, USA. Paper presented at the 56th International Astronautical controlled concurrently. Congress, Fukuoka, Japan, October 17-21, 2005. 1 In this paper we describe the design of a six- allows an arbitrarily large number of actuators to legged robot aimed at space explorative missions. be controlled concurrently and thus is particularly Strongly believing that a bio-inspired approach can suitable for legged locomotion control. Further ad- largely benefit the design of an autonomous legged vantages are ease of implementation, robustness and robot, we took explicitly inspiration from cockroach flexibility. experimental observations. Biological results inspired The implementation itself is advantageous since both the hexapod structure and the control system it just involves analog electronics. In space appli- architecture. cations, analog circuitry could hold many benefits. In order to replicate at least in part cockroach's Analog circuits provide a very high bandwidth sensor- extraordinary agility, each of the three leg pairs has a to-motor signal transformation and avoid any time- unique design: front legs and middle legs have 3 de- consuming conversion between analog and digital grees of freedom and a kind of pantograph mechanism signals; thus analog circuits allow actuator outputs aimed at facilitating obstacle climbing task, while rear to be rapidly modulated in response to sensor feed- legs have 2 degrees of freedom and a piston-like back, as needed for autonomous planetary explorers. design suitable for powerful forward thrusting. Our Most importantly, analog circuitry appears to be more main concern is on the mechanics of rear legs, that robust against space radiation. For example, Single appears to play a crucial role in obstacle overcoming Event Upset (SEU) (i.e. radiation-induced errors in and payload capability. Moreover Gregor I exhibits a microelectronic circuits caused when charged parti- sprawled posture, able to guarantee a statically stable cles lose energy by ionizing the medium through posture and thus a high margin of stability [2]. Dy- which they pass) in analog circuitry just causes a namical simulations and experimental tests prove that, graceful degradation in performance, while in digital thanks to the careful linear/rotational actuation and to circuitry can cause bit flips and therefore result in the sprawled posture, Gregor I is able to successfully catastrophic failures by placing the device into a test negotiate obstacles of considerable height. mode, halt, or undefined state [9]. This paper is organized as follows. In Section II we The methodology adopted in this paper for legged discuss basic biological observations in insects and locomotion control took its inspiration from the bio- we review some previous hexapod robots in literature. logical paradigm of Central Pattern Generator (CPG) Then, in Section III we describe the physical design, [3] and was firstly outlined in [4]. In insects, the while in Section IV we discuss the locomotion con- activation of the appropriate muscles in the legs and trol. Simulation results follow in Section V. In Section their coordination take place locally by means of VI we show the physical implementation and, finally, groups of neurons functionally organized in CPG in Section VII we draw our conclusions. modules. The basic units of the adopted artificial CPG are here nonlinear oscillators coupled together to form II. BIOLOGICAL INSPIRATION AND a network able to generate a pattern of synchroniza- LITERATURE REVIEW tion that is used to coordinate the robot actuators. Cellular Neural Network (CNN) paradigm, introduced Gregor I design is based on biological observations in [5], provides a framework for the implementation in insects. In this section, firstly we outline some of these nonlinear oscillators: each oscillator is simply of the most important results coming from insect viewed as a cell of a CNN. This technique has been experimental observations, with particular emphasis previously used to control the locomotion of sev- on the cockroach Blaberus Discoidalis. Then, we eral different bio-inspired robotic structures: simple review some previous hexapod robot in literature. hexapods, octopods and lamprey-like robots [6], [7]; here this technique is extended to the control of an A. Structure of Blaberus Discoidalis hexapod in which each leg pair has a unique design. Most important structural features of Blaberus Dis- A direct VLSI realization of the control system is coidalis from an engineering viewpoint are: possible: a chip for locomotion control implemented • leg structure; by a CNN-based CPG is introduced in [8]. • leg articulation; This approach, thanks to its intrinsic modularity, • body structure. 2 Each cockroach leg is divided into several seg- B. CPG and locomotion gaits ments. The leg segments from the most proximal to Most insects exhibit a hierarchical locomotion con- the most distal segment are called coxa, trochanter, trol and use a modular organization of the control femur, tibia and tarsus; the last one is indeed consti- elements. The activation of the appropriate muscles tuted by a series of foot joints. in the legs and their coordination take place locally The complex musculature coupled with complex by means of groups of neurons functionally orga- mechanics confers upon the joint between body and nized in modules called Central Pattern Generators coxa three degrees of freedom (DOF), much like that (CPG). The output signals of the CPG control directly of a ball and socket joint. The joints between the coxa the effector organs. Distinct periodic patterns of leg and trochanter, between the trochanter and femur, movements, called gaits, are due to patterns of neural and between the femur and tibia are, instead, simple activity within the CPG [14]. The CPG receives one DOF rotational joints. The joint between

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