DOCSLIB.ORG

Inverse Dynamics with Rigid Contact and Friction

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Inverse Dynamics with Rigid Contact and Friction Noname manuscript No. (will be inserted by the editor) Inverse Dynamics with Rigid Contact and Friction Samuel Zapolsky · Evan Drumwright Received: date / Accepted: date Abstract Inverse dynamics is used extensively in ro- ments gives an upper bound on the performance of such botics and biomechanics applications. In manipulator controllers in situ. For points of comparison, we assess and legged robots, it can form the basis of an effective performance on the same tasks with both error feed- nonlinear control strategy by providing a robot with back control and inverse dynamics control with virtual both accurate positional tracking and active compli- contact force sensing. ance. In biomechanics applications, inverse dynamics control can approximately determine the net torques Keywords inverse dynamics, rigid contact, impact, applied at anatomical joints that correspond to an ob- Coulomb friction served motion. In the context of robot control, using inverse dynam- ics requires knowledge of all contact forces acting on the 1 Introduction robot; accurately perceiving external forces applied to the robot requires filtering and thus significant time de- Inverse dynamics can be a highly effective method in lay. An alternative approach has been suggested in re- multiple contexts including control of robots and phys- cent literature: predicting contact and actuator forces ically simulated characters and estimating muscle forces simultaneously under the assumptions of rigid body dy- in biomechanics. The inverse dynamics problem, which namics, rigid contact, and friction. Existing such inverse entails computing actuator (or muscular) forces that dynamics approaches have used approximations to the yield desired accelerations, requires knowledge of all contact models, which permits use of fast numerical lin- \external" forces acting on the robot, character, or hu- ear algebra algorithms. In contrast, we describe inverse man. Measuring such forces is limited by the ability dynamics algorithms that are derived only from first to instrument surfaces and filter force readings, and principles and use established phenomenological mod- such filtering effectively delays force sensing to a degree els like Coulomb friction. unacceptable for real-time operation. An alternative is We assess these inverse dynamics algorithms in a to use contact force predictions, for which reasonable control context using two virtual robots: a locomoting agreement between models and reality have been ob- quadrupedal robot and a fixed-based manipulator grip- served (see, e.g., Aukes & Cutkosky, 2013). Formulat- ping a box while using perfectly accurate sensor data ing such approaches is technically challenging, however, from simulation. The data collected from these experi- because the actuator forces are generally coupled to the contact forces, requiring simultaneous solution. Samuel Zapolsky Inverse dynamics approaches that simultaneously George Washington University Dept. of Computer Science compute contact forces exist in literature. Though these [email protected] approaches were developed without incorporating all Evan Drumwright of the established modeling aspects (like complemen- George Washington University tarity) and addressing all of the technical challenges Dept. of Computer Science (like inconsistent configurations) of hard contact, these [email protected] methods have been demonstrated performing effectively 2 Samuel Zapolsky, Evan Drumwright on real robots. In contrast, this article focuses on for- We will assess these inverse dynamics algorithms in mulating inverse dynamics with these established model- the context of controlling a virtual locomoting robot ing aspects|which allows forward and inverse dynam- and a fixed-base manipulator robot. We will examine ics models to match|and addresses the technical chal- performance of error feedback and inverse dynamics lenges, including solvability. controllers with virtual contact force sensors for points of comparison. Performance will consider smoothness of torque commands, trajectory tracking accuracy, lo- 1.1 Invertibility of the rigid contact model comotion performance, and computation time. An obstacle to such a formulation has been the claim 1.3 Contributions that the rigid contact model is not invertible (Tod- orov, 2014), implying that inverse dynamics is unsolve- This paper provides the following contributions: able for multi-rigid bodies subject to rigid contact. If forces on the multi-body other than contact forces at 1. Proof that the coupled problem of computing in- state f q; q_ g are designated x and contact forces are verse dynamics-derived torques and contact forces designated y, then the rigid contact model (to be de- under the rigid body dynamics, non-impacting rigid scribed in detail in Section 2.3.3) yields the relationship contact, and Coulomb friction models (with linearized y = fq;q_(x). It is then true that there exists no left in- friction cone) is solvable in expected polynomial time. verse g(:) of f that provides the mapping x = gq;q_(y) 2. An algorithm that computes inverse dynamics-derived for y = fq;q_(x). However, this article will show that torques without torque chatter under the rigid body there does exist a right inverse h(:) of f such that, dynamics model and the rigid contact model assum- for hq;q_(y) = x, fq;q_(x) = y, and in Section 5 we ing no slip along the surfaces of contact, in expected will show that this mapping is computable in expected polynomial time. polynomial time. This article will use this mapping to 3. An algorithm that yields inverse dynamics-derived formulate inverse dynamics approaches for rigid con- torques without torque chatter under the rigid body tact with both no-slip constraints and frictional surface dynamics model and the rigid, non-impacting con- properties. tact model with Coulomb friction in exponential time in the number of points of contact, and hence a proof that this problem is no harder than NP-hard. 4. An algorithm that computes inverse dynamics-derived 1.2 Indeterminacy in the rigid contact model torques without torque chatter under the rigid body dynamics model and a rigid contact model with The rigid contact model is also known to be suscepti- Coulomb friction but does not enforce complemen- ble to the problem of contact indeterminacy, the pres- tarity conditions (again, see Sections 2.3.3 and 2.3.4), ence of multiple equally valid solutions to the contact in expected polynomial time. force-acceleration mapping. This indeterminacy is the factor that prevents strict invertibility and which, when These algorithms differ in their operating assump- used for contact force predictions in the context of in- tions. For example, the algorithms that enforce nor- verse dynamics, can result in torque chatter that is mal complementarity (to be described in Section 2.3) potentially destructive for physically situated robots. assume that all contacts are non-impacting; similarly, We show that computing a mapping from accelerations the algorithms that do not enforce complementarity as- to contact forces that evolves without harmful torque sume that bodies are impacting at one or more points chatter is no worse than NP-hard in the number of con- of contact. As will be explained in Section 3, control tacts modeled for Coulomb friction and can be calcu- loop period endpoint times do not necessarily coincide lated in polynomial time for the case of infinite (no-slip) with contact event times, so a single algorithm must friction. deal with both impacting and non-impacting contacts. This article also describes a computationally tract- It is an open problem of the effects of enforcing com- able approach for mitigating torque chatter that is based plementarity when it should not be enforced, or vice upon a rigid contact model without complementarity versa. The algorithms also possess various computa- conditions (see Sections 2.3.3 and 2.3.4). The model ap- tional strengths. As results of these open problems and pears to produce reasonable predictions: Anitescu (2006); varying computational strengths, we present multiple Drumwright & Shell (2010); Todorov (2014) have all algorithms to the reader as well as a guide (see Ap- used the model within simulation and physical artifacts pendix A) that details task use cases for these control- have yet to become apparent. lers. Inverse Dynamics with Rigid Contact and Friction 3 This work validates controllers based upon these ap- limit verification that controllers implementing these proaches in simulation to determine their performance inverse dynamics approaches for control work as ex- under a variety of measurable phenomena that would pected. These experiments also examine behavior when not be accessible on present day physically situated modeling assumptions break down. Section 8 analyzes robot hardware. Experiments indicating performance the findings from these experiments. differentials on present day (prototype quality) hard- ware might occur due to control forces exciting unmod- eled effects, for example. Results derived from simula- 1.5 The multi-body tion using ideal sensing and perfect torque control in- dicate the limit of performance for control software de- This paper centers around a multi-body, which is the scribed in this work. We also validate one of the more system of rigid bodies to which inverse dynamics is robust presented controllers on a fixed-base manipula- applied. The multi-body may come into contact with tor robot grasping task to demonstrate operation under “fixed” parts of the environment (e.g., solid ground)
Load more

Recommended publications
  • Chapter 3 Dynamics of Rigid Body Systems
    Rigid Body Dynamics Algorithms Roy Featherstone Rigid Body Dynamics Algorithms Roy Featherstone The Austrailian National University Canberra, ACT Austrailia Library of Congress Control Number: 2007936980 ISBN 978-0-387-74314-1 ISBN 978-1-4899-7560-7 (eBook) Printed on acid-free paper. @ 2008 Springer Science+Business Media, LLC All rights reserved. This work may not be translated or copied in whole or in part without the written permission of the publisher (Springer Science+Business Media, LLC, 233 Spring Street, New York, NY 10013, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights. 9 8 7 6 5 4 3 2 1 springer.com Preface The purpose of this book is to present a substantial collection of the most efficient algorithms for calculating rigid-body dynamics, and to explain them in enough detail that the reader can understand how they work, and how to adapt them (or create new algorithms) to suit the reader’s needs. The collection includes the following well-known algorithms: the recursive Newton-Euler algo- rithm, the composite-rigid-body algorithm and the articulated-body algorithm. It also includes algorithms for kinematic loops and floating bases.
    [Show full text]
  • Forward and Inverse Dynamics of a Six-Axis Accelerometer Based on a Parallel Mechanism
    sensors Article Forward and Inverse Dynamics of a Six-Axis Accelerometer Based on a Parallel Mechanism Linkang Wang 1 , Jingjing You 1,* , Xiaolong Yang 2, Huaxin Chen 1, Chenggang Li 3 and Hongtao Wu 3 1 College of Mechanical and Electronic Engineering, Nanjing Forestry University, Nanjing 210037, China; [email protected] (L.W.); [email protected] (H.C.) 2 School of Mechanical Engineering, Nanjing University of Science and Technology, Nanjing 210037, China; [email protected] 3 School of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China; [email protected] (C.L.); [email protected] (H.W.) * Correspondence: [email protected] Abstract: The solution of the dynamic equations of the six-axis accelerometer is a prerequisite for sensor calibration, structural optimization, and practical application. However, the forward dynamic equations (FDEs) and inverse dynamic equations (IDEs) of this type of system have not been completely solved due to the strongly nonlinear coupling relationship between the inputs and outputs. This article presents a comprehensive study of the FDEs and IDEs of the six-axis accelerometer based on a parallel mechanism. Firstly, two sets of dynamic equations of the sensor are constructed based on the Newton–Euler method in the configuration space. Secondly, based on the analytical solution of the sensor branch chain length, the coordination equation between the output signals of the branch chain is constructed. The FDEs of the sensor are established by combining the coordination equations and two sets of dynamic equations. Furthermore, by introducing generalized momentum and Hamiltonian function and using Legendre transformation, the vibration differential equations (VDEs) of the sensor are derived.
    [Show full text]
  • Existence and Uniqueness of Rigid-Body Dynamics With
    EXISTENCE AND UNIQUENESS OF RIGIDBODY DYNAMICS WITH COULOMB FRICTION Pierre E Dup ont Aerospace and Mechanical Engineering Boston University Boston MA USA ABSTRACT For motion planning and verication as well as for mo delbased control it is imp ortanttohavean accurate dynamic system mo del In many cases friction is a signicant comp onent of the mo del When considering the solution of the rigidb o dy forward dynamics problem in systems such as rob ots wecan often app eal to the fundamental theorem of dierential equations to prove the existence and uniqueness of the solution It is known however that when Coulomb friction is added to the dynamic equations the forward dynamic solution may not exist and if it exists it is not necessarily unique In these situations the inverse dynamic problem is illp osed as well Thus there is the need to understand the nature of the existence and uniqueness problems and to know under what conditions these problems arise In this pap er we show that even single degree of freedom systems can exhibit existence and uniqueness problems Next weintro duce compliance in the otherwise rigidb o dy mo del This has two eects First the forward and inverse problems b ecome wellp osed Thus we are guaranteed unique solutions Secondlyweshow that the extra dynamic solutions asso ciated with the rigid mo del are unstable INTRODUCTION To plan motions one needs to know what dynamic b ehavior will o ccur as the result of applying given forces or torques to the system In addition the simulation of motions is useful to verify motion
    [Show full text]
  • Newton Euler Equations of Motion Examples
    Newton Euler Equations Of Motion Examples Alto and onymous Antonino often interloping some obligations excursively or outstrikes sunward. Pasteboard and Sarmatia Kincaid never flits his redwood! Potatory and larboard Leighton never roller-skating otherwhile when Trip notarizes his counterproofs. Velocity thus resulting in the tumbling motion of rigid bodies. Equations of motion Euler-Lagrange Newton-Euler Equations of motion. Motion of examples of experiments that a random walker uses cookies. Forces by each other two examples of example are second kind, we will refer to specify any parameter in. 213 Translational and Rotational Equations of Motion. Robotics Lecture Dynamics. Independence from a thorough description and angular velocity as expected or tofollowa userdefined behaviour does it only be loaded geometry in an appropriate cuts in. An interface to derive a particular instance: divide and author provides a positive moment is to express to output side can be run at all previous step. The analysis of rotational motions which make necessary to decide whether rotations are. For xddot and whatnot in which a very much easier in which together or arena where to use them in two backwards operation complies with respect to rotations. Which influence of examples are true, is due to independent coordinates. On sameor adjacent joints at each moment equation is also be more specific white ellipses represent rotations are unconditionally stable, for motion break down direction. Unit quaternions or Euler parameters are known to be well suited for the. The angular momentum and time and runnable python code. The example will be run physics examples are models can be symbolic generator runs faster rotation kinetic energy.
    [Show full text]
  • Investigation Into the Utility of the MSC ADAMS Dynamic Software for Simulating
    Investigation into the Utility of the MSC ADAMS Dynamic Software for Simulating Robots and Mechanisms A thesis presented to The faculty of The Russ College of Engineering and Technology of Ohio University In partial fulfillment of the requirements for the degree Master of Science Xiao Xue May 2013 © 2013 Xiao Xue. All Rights Reserved 2 This thesis titled Investigation into the Utility of the MSC ADAMS Dynamic Software for Simulating Robots and Mechanisms by XIAO XUE has been approved for the Department of Mechanical Engineering and the Russ College of Engineering and Technology by Robert L. Williams II Professor of Mechanical Engineering Dennis Irwin Dean, Russ College of Engineering and Technology 3 ABSTRACT XUE, XIAO, M.S., May 2013, Mechanical Engineering Investigation into the Utility of the MSC ADAMS Dynamic Software for Simulating Robots and Mechanisms (pp.143) Director of Thesis: Robert L. Williams II A Slider-Crank mechanism, a 4-bar mechanism, a 2R planar serial robot and a Stewart-Gough parallel manipulator are modeled and simulated in MSC ADAMS/View. Forward dynamics simulation is done on the Stewart-Gough parallel manipulator; Inverse dynamics simulation is done on Slider-Crank mechanism, 2R planar serial robot and 4- bar mechanism. In forward dynamics simulation, the forces are applied in prismatic joints and the Stewart-Gough parallel manipulator is actuated to perform 4 different expected motions. The motion of the platform is measured and compared with the results in MATLAB. An example of inverse dynamics simulation is also performed on it. In the inverse dynamics simulation, the Slider-Crank mechanism and four-bar mechanism both run with constant input angular velocity and the actuating forces are measured and compared with the results in MATLAB.
    [Show full text]
  • An Inverse Dynamics Model for the Analysis, Reconstruction and Prediction of Bipedal Walking
    Pergamon J. Biomechanics, Vol. 28, No. 11, pp. 1369%1376,1995 Copyri& 0 1995 Elsevicr Science Ltd hinted in Great Britain. All rifits reserved 0021-9290/95 $9.50 + .oO 0021-9290(94)00185-J AN INVERSE DYNAMICS MODEL FOR THE ANALYSIS, RECONSTRUCTION AND PREDICTION OF BIPEDAL WALKING Bart Koopman, Henk .I. Grootenboer and Henk J. de Jongh University of Twente, Faculty of Mechanical Engineering, Laboratory of Biomedical Engineering, P.O. Box 217,750O AE Enschede, The Netherlands Abstract-Walkingis a constrainedmovement which may best be observed during the double stance phase when both feet contact the floor. When analyzing a measured movement with an inverse dynamics model, a violation of these constraints will always occur due to measuring errors and deviations of the segments model from reality, leading to inconsistent results. Consistency is obtained by implementing the constraints into the model. This makes it possible to combine the inverse dynamics model with optimization techniques in order to predict walking patterns or to reconstruct non-measured rotations when only a part of the three-dimensional joint rotations is measured. In this paper the outlines of the extended inverse dynamics method are presented, the constraints which detine walking are defined and the optimization procedure is described. The model is applied to analyze a normal walking pattern of which only the hip, knee and ankle flexions/extensions are measured. This input movement is reconstructed to a kinematically and dynam- ically consistent three-dimensional movement, and the joint forces (including the ground reaction forces) and joint moments of force, needed to bring about thts movement are estimated.
    [Show full text]
  • AN ALGORITHM for the INVERSE DYNAMICS of N-AXIS GENERAL MANIPULATORS USING KANE's EQUATIONS
    Computers Math. Applic. Vol. 17, No. 12, pp. 1545-1561, 1989 0097-4943/89 $3.00 + 0.00 Printed in Great Britain. All rights reserved Copyright © 1989 Pergamon Press plc AN ALGORITHM FOR THE INVERSE DYNAMICS OF n-AXIS GENERAL MANIPULATORS USING KANE'S EQUATIONS J. ANGELES and O. MA Department of Mechanical Engineering, Robotic Mechanical Systems Laboratory, McGill Research Centre for Intelligent Machines, McGill University, Montrral, Qurbec, Canada A. ROJAS National Autonomous University of Mexico, Villa Obregrn, Mexico (Received 4 April 1988) Abstract--Presented in this paper is an algorithm for the numerical solution of the inverse dynamics of robotic manipulators of the serial type, but otherwise arbitrary. The algorithm is applicable to manipulators containing n joints of the rotational or the prismatic type. For a given set of Hartenberg- Denavit and inertial parameters, as well as for a given trajectory in the space of joint coordinates, the algorithm produces the time histories of the n torques or forces required to drive the manipulator through the prescribed trajectory. The algorithm is based on Kane's dynamical equations of mechanical multibody systems. Moreover, the complexityof the algorithm presented here is lower than that of the most ett~cient inverse-dynamics algorithm reported in the literature. Finally, the applicability of the algorithm is illustrated with two fully solved examples. 1. INTRODUCTION The inverse dynamics of robotic manipulators has been approached with a variety of methods, the most widely applied being those based on the Newton-Euler (NE) and the Euler-Lagrange (EL) formulations [1-11]. Although essentially equivalent [12], the two said formulations regard the dynamics problems associated with rigid-link manipulators from very different viewpoints.
    [Show full text]
  • Forward and Inverse Dynamics Reconstruction of a Staged Rugby Tackle
    IRC-19-90 IRCOBI conference 2019 Forward and Inverse Dynamics Reconstruction of a Staged Rugby Tackle C. Weldon* & K. Archbold*, G. Tierney, D. Cazzola, E. Preatoni, K. Denvir, CK. Simms I. INTRODUCTION Sports participation is crucial to population health, but injuries carry short and long-term health consequences. In elite schoolboy rugby, injury incidence is 77/1000 match hours with concussions at 20/1000 hours [1], while in adult competitions the injured player proportion per year is around 50%, with tackles being the main injury cause [2]. However, little is known about contact forces/body kinematics leading to injuries in sporting collisions. Wearable sensors are promising, but challenges remain. Physics-based models with force predictions are subject to validation challenges [3-4]. We previously reported on staged rugby tackles recorded using a motion capture system [5]. Simulating these tackles using MADYMO (Helmond, The Netherlands) forward dynamics modelling highlighted challenges with the absence of muscle activation in the MADYMO pedestrian model [3]. Here our aim is to test whether combined inverse and forward dynamics modelling of a staged rugby tackle can yield a realistic estimate of tackle contact force. II. METHODS A combined forward and inverse dynamics approach was taken to estimate tackle contact forces without a direct load sensor, as impact force is challenging to measure in sporting collisions. To test this integrated approach, a staged tackle from [5] was chosen for forward and inverse dynamics model reconstruction. For forward dynamics, the MADYMO pedestrian model applied by [4] was used, with pelvis initial position, orientation and velocity and the principal joint DOFs and their time derivatives extracted from the Vicon motion capture data using the Plug-in-Gait Model [5].
    [Show full text]
  • Estimation of Ground Reaction Forces and Moments During Gait Using Only Inertial Motion Capture
    Article Estimation of Ground Reaction Forces and Moments During Gait Using Only Inertial Motion Capture Angelos Karatsidis 1,4,*, Giovanni Bellusci 1, H. Martin Schepers 1, Mark de Zee 2, Michael S. Andersen 3 and Peter H. Veltink 4 1 Xsens Technologies B.V., Enschede 7521 PR, The Netherlands; [email protected] (G.B.); [email protected] (H.M.S.) 2 Department of Health Science and Technology, Aalborg University, Aalborg 9220, Denmark; [email protected] 3 Department of Mechanical and Manufacturing Engineering, Aalborg University, Aalborg 9220, Denmark; [email protected] 4 Institute for Biomedical Technology and Technical Medicine (MIRA), University of Twente, Enschede 7500 AE, The Netherlands; [email protected] * Correspondence: [email protected]; Tel.: +31-53-489-3316 Academic Editor: Vittorio M. N. Passaro Received: 29 September 2016; Accepted: 28 December 2016; Published: 31 December 2016 Abstract: Ground reaction forces and moments (GRF&M) are important measures used as input in biomechanical analysis to estimate joint kinetics, which often are used to infer information for many musculoskeletal diseases. Their assessment is conventionally achieved using laboratory-based equipment that cannot be applied in daily life monitoring. In this study, we propose a method to predict GRF&M during walking, using exclusively kinematic information from fully-ambulatory inertial motion capture (IMC). From the equations of motion, we derive the total external forces and moments. Then, we solve the indeterminacy problem during double stance using a distribution algorithm based on a smooth transition assumption. The agreement between the IMC-predicted and reference GRF&M was categorized over normal walking speed as excellent for the vertical (r = 0.992, rRMSE = 5.3%), anterior (r = 0.965, rRMSE = 9.4%) and sagittal (r = 0.933, rRMSE = 12.4%) GRF&M components and as strong for the lateral (r = 0.862, rRMSE = 13.1%), frontal (r = 0.710, rRMSE = 29.6%), and transverse GRF&M (r = 0.826, rRMSE = 18.2%).
    [Show full text]
  • A Unified Method for Solving Inverse, Forward, and Hybridmanipulator
    A Unified Method for Solving Inverse, Forward, and HybridManipulator Dynamics using Factor Graphs Mandy Xie, Frank Dellaert (presented by Dominic Guri) Introduction 1. Inverse Dynamics I solve for forces & torques (for control and planning) for a desired trajectories (positions, velocities, and accelerations) I often used by Newton-Euler N_E — very efficient, and recursion-based 2. Forward Dynamics I solves joint accelerations when torques are applied at known positions and velocities I different from simulation problem with requires motion integration to obtain positions and velocities I solutions: I Composite-Rigid-Body Algorithm (CRBA) — can be numerically unstable I Articulated-Body Algorithm (ABA) — takes advantage of sparsity 3. Hybrid Dynamics (partially both inverse and forward) I solves for unknown forces and accelerations, given forces at some joints and accelerations at other joints NB: all the above methods do not handle kinematic loops NB: rare for a single algorithm to solve all 3 problems — proposed methods use filtering and smoothing Dynamics Problems: Figure 1: QUT Robot Academy Existing Methods 1. Inverse Dynamics I Recursive Newton-Euler [42,21] I others [49, 57] 2. Forward Dynamics I Composite-Rigid-Body Algorithm (CRBA) [60,20] I Articulated-Body Algorithm (ABA) [18] 3. Hybrid Dynamics (partially both inverse and forward) I Articulated-Body Hybrid Dynamics Algorithm [19] 4. Kinematic Loops I [19] I Rodriguez [52,51] — filtering and smoothing based, inverse & forward I applied to loops in [53] I Rodriguez [54] ⇒ I Jain [33] — serial chain dynamics I Ascher [4] — CRBA & ABA as elimination methods for solving forward dynamics Contributions I factor graph as a unified description of classical dynamics algorithms and new algorithms (graph theory-based ⇒ sparse linear systems) 1.
    [Show full text]
  • Inverse Dynamics Vs. Forward Dynamics in Direct Transcription Formulations for Trajectory Optimization
    Inverse Dynamics vs. Forward Dynamics in Direct Transcription Formulations for Trajectory Optimization Henrique Ferrolho1, Vladimir Ivan1, Wolfgang Merkt2, Ioannis Havoutis2, Sethu Vijayakumar1 Abstract— Benchmarks of state-of-the-art rigid-body dynam- ics libraries report better performance solving the inverse dynamics problem than the forward alternative. Those bench- marks encouraged us to question whether that computational advantage would translate to direct transcription, where cal- culating rigid-body dynamics and their derivatives accounts for a significant share of computation time. In this work, we implement an optimization framework where both approaches for enforcing the system dynamics are available. We evaluate the performance of each approach for systems of varying complexity, for domains with rigid contacts. Our tests reveal that formulations using inverse dynamics converge faster, re- quire less iterations, and are more robust to coarse problem discretization. These results indicate that inverse dynamics should be preferred to enforce the nonlinear system dynamics in simultaneous methods, such as direct transcription. I. INTRODUCTION Fig. 1: Snapshots of the humanoid TALOS [2] jumping. Direct transcription [1] is an effective approach to for- mulate and solve trajectory optimization problems. It works constraints are usually at the very core of optimal control by converting the original trajectory optimization problem formulations, and require computing rigid-body dynamics (which is continuous in time) into a numerical optimization and their derivatives—which account for a significant portion problem that is discrete in time, and which in turn can be of the optimization computation time. Therefore, it is of solved using an off-the-shelf nonlinear programming (NLP) utmost importance to use an algorithm that allows to compute solver.
    [Show full text]
  • A Foot/Ground Contact Model for Biomechanical Inverse Dynamics Analysis
    A foot/ground contact model for biomechanical inverse dynamics analysis Romain Van Hulle∗, Cédric Schwartz∗, Vincent Denoël∗, Jean-Louis Croisier∗, Bénédicte Forthomme∗, Olivier Brüls∗ Laboratory of Human Motion Analysis, University of Liège, Belgium Abstract The inverse dynamics simulation of the musculoskeletal system is a common method to understand and analyse human motion. The ground reaction forces can be accurately estimated by experimental measurements using force platforms. However, the number of steps is limited by the number of force platforms available in the laboratory. Several numerical methods have been proposed to estimate the ground reaction forces without force platforms, i.e., solely based on kinematic data combined with a model of the foot-ground contact. The purpose of this work is to provide a more efficient method, using a unilaterally constrained model of the foot at the center of pressure to compute the ground reaction forces. The proposed model does not require any data related with the compliance of the foot- ground contact and is kept as simple as possible. The indeterminacy in the force estimation is handled using a least square approach with filtering. The relative root mean square error (rRMSE) between the numerical estimations and experimental measurements are 4:1% for the vertical component of the ground reaction forces (GRF), 11:2% for the anterior component and 5:3% for the ground reaction moment (GRM) in the sagittal plane. Keywords: Musculoskeletal model, Inverse dynamics, Ground reaction forces, Contact constraint, Rigid foot model 1. Introduction Inverse dynamics simulation based on musculoskeletal models is a common and important method to study and understand the human motion as it can estimate joint torques based on experimental motion analysis data.
    [Show full text]