University of Pennsylvania ScholarlyCommons Departmental Papers (ESE) Department of Electrical & Systems Engineering September 2008 Integrating a Hierarchy of Simulation Tools for Legged Robot Locomotion Andrew Slatton Georgia Institute of Technology, [email protected] Yang Ding Georgia Institute of Technology, [email protected] P B. Umbanhowar [email protected] Daniel Goldman Georgia Institute of Technology, [email protected] Galen C. Haynes University of Pennsylvania, [email protected] See next page for additional authors Follow this and additional works at: https://repository.upenn.edu/ese_papers Recommended Citation Andrew Slatton, Yang Ding, P B. Umbanhowar, Daniel Goldman, Galen C. Haynes, Haldun Komsuoglu, Daniel E. Koditschek, and Daniel Cohen, "Integrating a Hierarchy of Simulation Tools for Legged Robot Locomotion", . September 2008. This paper is posted at ScholarlyCommons. https://repository.upenn.edu/ese_papers/474 For more information, please contact [email protected]. Integrating a Hierarchy of Simulation Tools for Legged Robot Locomotion Abstract We are interested in the development of a variety of legged robot platforms intended for operation in unstructured outdoor terrain. In such settings, the traditions of rational engineering design, driven by analytically informed and computationally assisted studies of robot-environment models, remain ineffective due to the complexity of both the robot designs and the terrain in which they must operate. Instead, empirical trial and error often drives the necessary incremental and iterative design process, hence the development of such robots remains expensive both in time and cost, and is often closely dependent upon the substrate properties of the locomotion terrain. This paper describes a series of concurrent but increasingly coordinated software development efforts that aim to diminish the gap between easily interfaced and physically sound computational models of a real robot’s operation in a complex natural environment. We describe a robot simulation environment in which simple robot modifications can be easily prototyped along and “played” into phenomenological models of contact mechanics. We particularly focus on the daunting but practically compelling example of robot feet interacting granular media, such as gravel or sand, offering a brief report of our progress in deriving and importing physically accurate but computationally tractable phenomenological substrate models into the robot execution simulation environment. With a goal of integration for future robot prototyping simulations, we review the prospects for diminishing the gap between the integrated computational models and the needs of physical platform development. Keywords Simulation Tools, Legged Robot Locomotion Author(s) Andrew Slatton, Yang Ding, P B. Umbanhowar, Daniel Goldman, Galen C. Haynes, Haldun Komsuoglu, Daniel E. Koditschek, and Daniel Cohen This other is available at ScholarlyCommons: https://repository.upenn.edu/ese_papers/474 Integrating a Hierarchy of Simulation Tools for Legged Robot Locomotion Andrew Slatton∗, Daniel Cohen∗, Yang Ding∗, Paul B. Umbanhowar∗, Daniel I. Goldman∗, G. Clark Haynesy, Haldun Komsuogluy, and Daniel E. Koditscheky ∗School of Physics yElectrical and Systems Engineering Georgia Institute of Technology University of Pennsylvania 837 State St NW, Atlanta, GA 30332 200 S. 33rd St, Philadelphia, PA 19104 Abstract— We are interested in the development of a va- the previous generation of rigid members joined along fixed riety of legged robot platforms intended for operation in seams. Moreover, substrate properties, for which there may unstructured outdoor terrain. In such settings, the traditions of not even be clear first principles models, and contact mechan- rational engineering design, driven by analytically informed and computationally assisted studies of robot-environment models, ics play a central role in determining ultimate performance. remain ineffective due to the complexity of both the robot A flexible and programmable simulation environment that designs and the terrain in which they must operate. Instead, captures both the robot physics as well as the substrate empirical trial and error often drives the necessary incremental properties would be a great asset further speeding the design- and iterative design process, hence the development of such test cycle and guiding the physical construction process. robots remains expensive both in time and cost, and is often closely dependent upon the substrate properties of the locomo- Although we have gained substantial benefit from running tion terrain. This paper describes a series of concurrent but different types of simulations at different points in the design increasingly coordinated software development efforts that aim cycle, it remains impossible to draw any reliable predictions to diminish the gap between easily interfaced and physically about the likely success of a given robot in the common, sound computational models of a real robot’s operation in a natural environments we target. This paper will explore complex natural environment. We describe a robot simulation environment in which simple robot modifications can be easily the various components of the simulation tools we have prototyped along and “played” into phenomenological models developed, and review the prospects for diminishing the gap of contact mechanics. We particularly focus on the daunting between the integrated computational models and the needs but practically compelling example of robot feet interacting of physical platform development. granular media, such as gravel or sand, offering a brief report We introduce “SimLib”1, an environment conducive to of our progress in deriving and importing physically accurate but computationally tractable phenomenological substrate mod- the rapid prototyping of kinematic chain simulations. This els into the robot execution simulation environment. With a environment allows a robot behavior developer to quickly test goal of integration for future robot prototyping simulations, modifications to robot morphology, as well as to the contact we review the prospects for diminishing the gap between the models to simulate interactions between feet and surfaces. integrated computational models and the needs of physical With an API identical to that of a series of physical robots platform development. [3], [7], [9], [2], we provide a tool for robot programmers to design behaviors with real-time, interactive simulations, by I. INTRODUCTION reducing simulator accuracy in favor of speed, or to batch run We have been collaborating with others [1] on the de- computationally-intensive simulations of higher accuracy. velopment of a variety of legged robot platforms intended Here, the problem goes beyond questions of computational for operation in unstructured outdoor terrain such as vertical efficiency and speedup of simulation for rapid prototyping. (building exteriors, trees, etc.) [2] and broken, unstable level Locomotion of a robot over complex ground requires novel ground [3]. Mobile robot design for such natural outdoor models of the foot-surface interaction which are only now be- settings defies purely analytical approaches. Successful im- ing developed and validated. We focus on common substrates plementation [4], [5], [6], [7] requires repeated iteration of like gravel or sand that are characterized by small grain inter- passive mechanical construction and actuated system com- actions between myriad solid particles. Such granular media ponents, ultimately driving the design cost both financially are challenging to a robot because the material can develop and in time. Rapid prototyping techniques such as SDM [5] fluid-like and solid-like forces in response to penetration of and modular electromechanical designs [3], [8] enlarge the accessible design space and accelerate the iterated design- 1SimLib is part of the RHexLib control software, originally developed by the DARPA RHex project [4] http://www.rhex.org and continued within build-test cycle, and these novel materials may be more the DARPA RiSE project [1] http://www.riserobot.org, and is available by challenging to model in computationally tractable form than request from the authors. Future plans exist for eventual public distribution. to simulate at the exact time-of-contact between two surfaces allows relatively straightforward incorporation of customized robot-surface interaction models, as will be discussed below. A. Creature Description Language The CDL is an abstraction of the low-level SimLib sim- ulation library, providing a simple language, programmed using the common tools of lex [10] and yacc [11] , that is designed specifically to describe multi-body linkages. It has support for an assortment of shapes, joints, and other common features of robot simulations. The language syntax facilitates building a tree representation of a simulation, in which objects are defined by their connections to previous elements. The connections between objects include defini- tions of linkages using rigid transforms or by defining joints. Fig. 1. Organization of SimLib within our robot control software. SimLib Using these language features, the CDL allows a simulation exposes an API identical to a physical robot. A run-time interface translates designer to describe complex robots using much less code robot translates motor commands and sensors readings to and from the than would be required if interfacing directly with a C++